Change Lengthable trait method names to into_len/from_len

std::convert::From<usize> cannot be used here because we cannot
implement bool<->usize conversions, due to Rust's orphan rules:

http://smallcultfollowing.com/babysteps/blog/2015/01/14/little-orphan-impls/
"prevent you from implementing external traits for external types"

Nonetheless, Lengthable used the same methods as From. This is allowed
but can require disambiguation if both are used, no longer strictly
needed for #140 but to reduce confusion and improve clarity, renamed
`from` to `from_len` and `into` to `into_len`.

src/protocol/mod.rs

@@ -446,8 +446,8 @@ impl Serializable for UUID {
 
 
 pub trait Lengthable : Serializable + Copy + Default {
-    fn into(self) -> usize;
-    fn from(_: usize) -> Self;
+    fn into_len(self) -> usize;
+    fn from_len(_: usize) -> Self;
 }
 
 pub struct LenPrefixed<L: Lengthable, V> {
@@ -467,7 +467,7 @@ impl <L: Lengthable, V: Default>  LenPrefixed<L, V> {
 impl <L: Lengthable, V: Serializable>  Serializable for LenPrefixed<L, V> {
     fn read_from<R: io::Read>(buf: &mut R) -> Result<LenPrefixed<L, V>, Error> {
         let len_data: L = Serializable::read_from(buf)?;
-        let len: usize = len_data.into();
+        let len: usize = len_data.into_len();
         let mut data: Vec<V> = Vec::with_capacity(len);
         for _ in 0..len {
             data.push(Serializable::read_from(buf)?);
@@ -479,7 +479,7 @@ impl <L: Lengthable, V: Serializable>  Serializable for LenPrefixed<L, V> {
     }
 
     fn write_to<W: io::Write>(&self, buf: &mut W) -> Result<(), Error> {
-        let len_data: L = L::from(self.data.len());
+        let len_data: L = L::from_len(self.data.len());
         len_data.write_to(buf)?;
         let data = &self.data;
         for val in data {
@@ -523,7 +523,7 @@ impl <L: Lengthable>  LenPrefixedBytes<L> {
 impl <L: Lengthable>  Serializable for LenPrefixedBytes<L> {
     fn read_from<R: io::Read>(buf: &mut R) -> Result<LenPrefixedBytes<L>, Error> {
         let len_data: L = Serializable::read_from(buf)?;
-        let len: usize = len_data.into();
+        let len: usize = len_data.into_len();
         let mut data: Vec<u8> = Vec::with_capacity(len);
         buf.take(len as u64).read_to_end(&mut data)?;
         Result::Ok(LenPrefixedBytes {
@@ -533,7 +533,7 @@ impl <L: Lengthable>  Serializable for LenPrefixedBytes<L> {
     }
 
     fn write_to<W: io::Write>(&self, buf: &mut W) -> Result<(), Error> {
-        let len_data: L = L::from(self.data.len());
+        let len_data: L = L::from_len(self.data.len());
         len_data.write_to(buf)?;
         buf.write_all(&self.data[..])?;
         Result::Ok(())
@@ -557,42 +557,42 @@ impl <L: Lengthable> fmt::Debug for LenPrefixedBytes<L> {
 }
 
 impl Lengthable for bool {
-    fn into(self) -> usize {
+    fn into_len(self) -> usize {
         if self  { 1 } else { 0 }
     }
 
-    fn from(u: usize) -> bool {
+    fn from_len(u: usize) -> bool {
         u != 0
     }
 }
 
 
 impl Lengthable for u8 {
-    fn into(self) -> usize {
+    fn into_len(self) -> usize {
         self as usize
     }
 
-    fn from(u: usize) -> u8 {
+    fn from_len(u: usize) -> u8 {
         u as u8
     }
 }
 
 impl Lengthable for i16 {
-    fn into(self) -> usize {
+    fn into_len(self) -> usize {
         self as usize
     }
 
-    fn from(u: usize) -> i16 {
+    fn from_len(u: usize) -> i16 {
         u as i16
     }
 }
 
 impl Lengthable for i32 {
-    fn into(self) -> usize {
+    fn into_len(self) -> usize {
         self as usize
     }
 
-    fn from(u: usize) -> i32 {
+    fn from_len(u: usize) -> i32 {
         u as i32
     }
 }
@@ -603,11 +603,11 @@ impl Lengthable for i32 {
 pub struct VarInt(pub i32);
 
 impl Lengthable for VarInt {
-    fn into(self) -> usize {
+    fn into_len(self) -> usize {
         self.0 as usize
     }
 
-    fn from(u: usize) -> VarInt {
+    fn from_len(u: usize) -> VarInt {
         VarInt(u as i32)
     }
 }
@@ -666,11 +666,11 @@ impl fmt::Debug for VarInt {
 pub struct VarShort(pub i32);
 
 impl Lengthable for VarShort {
-    fn into(self) -> usize {
+    fn into_len(self) -> usize {
         self.0 as usize
     }
 
-    fn from(u: usize) -> VarShort {
+    fn from_len(u: usize) -> VarShort {
         VarShort(u as i32)
     }
 }
@@ -725,11 +725,11 @@ impl fmt::Debug for VarShort {
 pub struct VarLong(pub i64);
 
 impl Lengthable for VarLong {
-    fn into(self) -> usize {
+    fn into_len(self) -> usize {
         self.0 as usize
     }
 
-    fn from(u: usize) -> VarLong {
+    fn from_len(u: usize) -> VarLong {
         VarLong(u as i64)
     }
 }