fix: parse int and uint using the native integer width

[jakezwang]

The default int and uint parsers reject valid values above 32 bits even on 64-bit platforms. Use strconv.IntSize for these native types so parsing accepts their full range while still rejecting overflow.

Add public parsing tests for signed min/max, unsigned max and overflow, with expectations for both 32-bit and 64-bit platforms. Full make ci passes on Go 1.18.10, 1.26.6 and 1.27.1; the tests also cross-compile for Linux/386.