Add support for a modded block: rockwool from Thermal Expansion (#153)

The first in support for modded content, a simple custom block: "rockwool" from the Thermal Expansion and Thermal Foundation mods for Forge:

https://ftb.gamepedia.com/Rockwool_(Thermal_Expansion_3)

This makes use of the Forge handshake (#88 #134 #144), matching the mod block names from the negotiation to numeric identifiers in the world to steven_blocks. Rockwool was chosen due to ease of implementation, it looks like wool from vanilla (except is fire-proof), and by supporting it the groundwork necessary is laid for more sophisticated mod support.

Tested with Thermal Expansion on 1.7.10, 1.10.2 (FTB Beyond), and 1.12.2 Forge servers.

* Add `modid` macro token, skipped from vanilla mappings

* Add ThermalExpansionRockwool block (1.7.10)

* Register modded blocks by modid->[data], and lookup block metadata

* Save block IDs from ModIdData/RegistryData to World modded_block_ids

* Add namespaced mod ids for ModIdData, \u{1}=block \u{2}=item

* Add ThermalFoundation's Rockwool (1.12.2)

protocol/src/protocol/forge.rs

@@ -85,6 +85,9 @@ impl Serializable for ModIdMapping {
     }
 }
 
+pub static BLOCK_NAMESPACE: &'static str = "\u{1}";
+pub static ITEM_NAMESPACE: &'static str = "\u{2}";
+
 #[derive(Debug)]
 pub enum FmlHs {
     ServerHello {