Various refactors that reduce compilation latency

[Drvi]

• Now we have a precompile.jl file
• The dict that stores all parsed definitions used be type-annotated for AbstractProtoType, now it has much more precise Union{MessageType, EnumType, ServiceType} type annotation.
• We used to parse enums into a NamedTuple -- this meant creating a new type for each parsed enum. Now we store the enumerated names and integer values in plain arrays.
• All Generators and Iterators.{map,flatten} that were used in type-unstable context were replaced with custom iteration/traversal functions. The type unstable context was usually iterating/traversing fields with unpredictable types.
• Couple of @nospecialized were sprinkled around (mostly avoiding specialization of parametrized field types which didn't really do anything type-specific)
• All print calls with multiple input arguments were simplified to only be given string inputs

master:

julia> import ProtocolBuffers as PB
[ Info: Precompiling ProtocolBuffers [33b38db7-f37a-4066-8d88-48be842fb53f]

julia> @time  PB.protojl("test_protos/google/protobuf/unittest.proto", ["./test/", "./test/test_protos/"], "dev/tst2", add_kwarg_constructors=true);
  3.893629 seconds (10.20 M allocations: 531.138 MiB, 4.36% gc time, 98.44% compilation time)

PR:

julia> import ProtocolBuffers as PB

julia> @time  PB.protojl("test_protos/google/protobuf/unittest.proto", ["./test/", "./test/test_protos/"], "dev/tst2", add_kwarg_constructors=true);
  1.074081 seconds (1.91 M allocations: 100.595 MiB, 1.30% gc time, 98.22% compilation time)

[codecov[bot]]

Codecov Report

Merging #9 (4b2f7fe) into master (d464ab7) will decrease coverage by 3.14%. The diff coverage is 87.86%.

:exclamation: Current head 4b2f7fe differs from pull request most recent head 7721d7d. Consider uploading reports for the commit 7721d7d to get more accurate results

@@            Coverage Diff             @@
##           master       #9      +/-   ##
==========================================
- Coverage   89.52%   86.38%   -3.15%     
==========================================
  Files          22       23       +1     
  Lines        2502     2629     +127     
==========================================
+ Hits         2240     2271      +31     
- Misses        262      358      +96     

Impacted Files	Coverage Δ
src/ProtocolBuffers.jl	85.71% <ø> (ø)
src/codegen/modules.jl	79.74% <ø> (-3.59%)	:arrow_down:
src/precompile.jl	0.00% <0.00%> (ø)
src/codegen/encode_methods.jl	68.59% <73.46%> (-22.62%)	:arrow_down:
src/lexing/Lexers.jl	87.27% <77.77%> (-3.02%)	:arrow_down:
src/codegen/CodeGenerators.jl	84.84% <83.87%> (-15.16%)	:arrow_down:
src/parsing/utils.jl	73.40% <97.14%> (+0.16%)	:arrow_up:
src/codegen/types.jl	94.05% <97.43%> (-0.84%)	:arrow_down:
src/codegen/decode_methods.jl	98.88% <100.00%> (+0.05%)	:arrow_up:
src/codegen/defaults.jl	96.38% <100.00%> (ø)
... and 14 more

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