Add 18w50a (451) multiprotocol support

[iceiix]

A step towards https://github.com/iceiix/steven/issues/72 1.14 protocol support

https://wiki.vg/Pre-release_protocol (https://wiki.vg/index.php?title=Pre-release_protocol&oldid=14491)
https://minecraft.gamepedia.com/18w50a
https://minecraft.gamepedia.com/1.14
https://pokechu22.github.io/Burger/18w50a.html

[iceiix]

Not too many protocol changes, those documented at least. But with the correct IDs it crashes parsing 0x56 Declare Recipes:

thread '' panicked at 'unrecognized recipe type: blasting', src/protocol/packet.rs:2372:18

There is a new type for https://minecraft.gamepedia.com/Blast_Furnace, and presumably https://minecraft.gamepedia.com/Smoker, need to parse these. No 18w50a or 1.14 on https://github.com/PrismarineJS/minecraft-data/tree/master/data/pc yet so flying blind.

[iceiix]

Got past the recipes packet, next is 0x22 chunk data:

thread 'main' panicked at 'Err: IOError(Custom { kind: InvalidData, error: StringError("stream did not contain valid UTF-8") })', src/server/mod.rs:437:33

https://wiki.vg/Pre-release_protocol#Chunk_Data

after primary bitmask, added a new heightmaps NBT field

[iceiix]

Chunk data packet parses, but crashes out of bounds:

thread 'main' panicked at 'index out of bounds: the len is 4 but the index is 4', libcore/slice/mod.rs:2448:10
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: <std::panicking::begin_panic::PanicPayload<A> as core::panic::BoxMeUp>::get
             at libstd/panicking.rs:476
   5: std::panicking::continue_panic_fmt
             at libstd/panicking.rs:390
   6: std::panicking::try::do_call
             at libstd/panicking.rs:325
   7: core::ptr::drop_in_place
             at libcore/panicking.rs:77
   8: core::ptr::drop_in_place
             at libcore/panicking.rs:59
   9: <usize as core::slice::SliceIndex<[T]>>::index
             at libcore/slice/mod.rs:2448
  10: core::slice::<impl core::ops::index::Index<I> for [T]>::index
             at libcore/slice/mod.rs:2316
  11: <alloc::vec::Vec<T> as core::ops::index::Index<I>>::index
             at liballoc/vec.rs:1653
  12: steven::types::bit::map::Map::get
             at src/types/bit/map.rs:99
  13: steven::world::World::load_chunk19
             at src/world/mod.rs:889
  14: steven::server::Server::on_chunk_data_heightmap
             at src/server/mod.rs:1126
  15: steven::server::Server::entity_tick
             at src/server/mod.rs:386
  16: steven::server::Server::tick
             at src/server/mod.rs:349
  17: steven::main
             at src/main.rs:277
  18: std::rt::lang_start::{{closure}}
             at libstd/rt.rs:74
  19: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  20: macho_symbol_search
             at libpanic_unwind/lib.rs:102
  21: std::alloc::default_alloc_error_hook
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  22: std::rt::lang_start
             at libstd/rt.rs:74

                for bi in 0 .. 4096 {
                    let id = m.get(bi);

[iceiix]

Chunk section format, the data field, also changed: https://wiki.vg/Pre-release_protocol#Chunk_Section_structure - added block count short, removed block/sky light (since it moved to Update Light packet)

[iceiix]

Now able to connect and see chunks, but since lighting moved to a separate packet and it isn't yet handled, lighting is incorrect. Also seems some missing block assignments:

[iceiix]

Works well enough for a snapshot, and 19w02a (452) is now out, so merging this for now