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