Handle too-big HashMap capacity
Handle reading of invalid enum values
Small cleanup of float reading
Use reader/writer instead of inStream/outStream
Zig 0.7.1
Use Arch image; Alpine doesn't have Zig 0.7.0 yet
Port everything so `zig build test` works on 0.7.0

The earlier commit got what I was using working on 0.7.0, but I forgot
to run the test suite to ensure everything was updated. Now I have run
it and all of the tests work.

- The varint tests were failing because the `readVarInt` functions are
  private. I moved these tests to `src/bare.zig` because I figure these
  are internal details that shouldn't be made public.
Update for zig 0.7.0
Add test for duplicate enum values
Add build manifest
Better compile error messages
Test non-slice pointers
Tests for bare unions
Add .gitignore
Test compile errors
Add note on hash maps to README
Try to support future Zig version[s]
zig fmt
Clean up `void` handling

As it's only supported in tagged unions, might as well special-case it there.
Test tagged union with active void member
Handle maps with 0 items