Make wiretypes into julia types and specialize methods on these types

[neveritt]

This PR removes the need for the WIRETYPES lookup table by specializing the lookup based on Julia types instead.
The lookup table is replace by wire_type, _read_value and _write_value functions.
This has a significant performance improvement for writing, but does not seem affect reading.

For example:

julia> using ProtoBuf, BenchmarkTools
julia> using ProtoBuf.google.protobuf

julia> dv = DoubleValue(value=64.0)
julia> dvmeta = ProtoBuf.meta(typeof(dv))
julia> @benchmark writeproto(iob, dv, dvmeta) setup = (iob = PipeBuffer();)
BenchmarkTools.Trial: 10000 samples with 308 evaluations.
 Range (min … max):  279.825 ns …  14.253 μs  ┊ GC (min … max): 0.00% … 97.90%
 Time  (median):     283.951 ns               ┊ GC (median):    0.00%
 Time  (mean ± σ):   297.450 ns ± 188.815 ns  ┊ GC (mean ± σ):  0.87% ±  1.37%

compared to

@benchmark writeproto(iob, dv, dvmeta) setup = (iob = PipeBuffer();)
BenchmarkTools.Trial: 10000 samples with 193 evaluations.
 Range (min … max):  502.933 ns …   8.296 μs  ┊ GC (min … max): 0.00% … 93.58%
 Time  (median):     512.648 ns               ┊ GC (median):    0.00%
 Time  (mean ± σ):   533.061 ns ± 106.134 ns  ┊ GC (mean ± σ):  0.15% ±  0.94%

in the previous version.

In order to achieve the specialization for the types that has several serializations, e.g. (int64, sint64, sfixed64)
two types SignedNumber and FixedSizeNumber are introduced to carry extra information about serialization.

I had to rearrange some methods to make the dispatch work properly, which may make the PR a bit hard to read.

[tanmaykm]

LGTM. Thanks!