32bit support

[samysweb]

This pull request intends to make ProtoBuf.jl and its CI compatible to 32-bit architectures (x86).

For CI I tried to run ProtoBuf.jl on a 32-bit system and noticed a bug due to the way LengthDelimitedProtoDecoder implements endpos:
Int is platform-dependent in Julia, and hence this corresponds to Int32 in 32-bit systems.
Conversely, IOBuffer (what my test uses) uniformly returns Int64 for position independently of the platform.

This issue also becomes visible when activating the CI of ProtoBuf.jl for x86 although for a different reason (UInt32 not converted into Int32).
I propose to uniformly use Int64 as an endmarker and to perform explicit conversion (see suggested changes to Codecs.jl and decode.jl.
I've also updated the tests in test_protojl.jl to make the choice of Integer type explicit (which is lateron checked) and to account for the fewer allocations on x86 systems in test_perf.jl.

With these changes, the CI now runs flawlessly everywhere except for x86 Julia nightly.
Where a Dict seems to be encoded in the opposite order for Julia Nightly on x86 (?).
Note that two tests fail and that expected/observed outputs are swapped between the two tests...
I am open to fixing this remaining bug, but would require your guidance on whether this is an implementation bug or a limitation of the current test?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.99%. Comparing base (79df484) to head (6370c6e).
⚠️ Report is 13 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #275   +/-   ##
=======================================
  Coverage   93.98%   93.99%           
=======================================
  Files          25       25           
  Lines        3011     3012    +1     
=======================================
+ Hits         2830     2831    +1     
  Misses        181      181           

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[samysweb]

As expected, the two failing tests concern nightly for x86, note that the two failing tests have swapped expectations:

encoded payload 1: Test Failed at D:\a\ProtoBuf.jl\ProtoBuf.jl\test\test_encode.jl:90
  Expression: elbytes == elexpected
   Evaluated: UInt8[0x0a, 0x01, 0x62, 0x12, 0x01, 0x61] == UInt8[0x0a, 0x01, 0x63, 0x12, 0x01, 0x64]
[...]
encoded payload 2: Test Failed at D:\a\ProtoBuf.jl\ProtoBuf.jl\test\test_encode.jl:90
  Expression: elbytes == elexpected
   Evaluated: UInt8[0x0a, 0x01, 0x63, 0x12, 0x01, 0x64] == UInt8[0x0a, 0x01, 0x62, 0x12, 0x01, 0x61]

I.e. elexpected of encoded payload 1 corresponds to elbytes of encoded payload 2 and conversely.