Added cross platform tests workflow

[VictorKoenders]

This PR uses cargo cross to cross compile a whole bunch of targets, and simply runs cargo test on those platforms.

I've added the following targets, based on this table, but removed all -musl targets as these should ™️ be the same as their -gnu counterparts.

• aarch64-unknown-linux-gnu
• arm-unknown-linux-gnueabi
• arm-unknown-linux-gnueabihf
• armv5te-unknown-linux-gnueabi
• armv7-unknown-linux-gnueabihf
• i586-unknown-linux-gnu
• i686-pc-windows-gnu
• i686-unknown-linux-gnu
• mips-unknown-linux-gnu
• mips64-unknown-linux-gnuabi64
• mips64el-unknown-linux-gnuabi64
• mipsel-unknown-linux-gnu
• powerpc-unknown-linux-gnu
• powerpc64-unknown-linux-gnu
• powerpc64le-unknown-linux-gnu
• riscv64gc-unknown-linux-gnu
• sparc64-unknown-linux-gnu
• x86_64-pc-windows-gnu
• x86_64-unknown-linux-gnu

I've opened #535 as a tracking issue for the platforms that don't build

[codecov]

Codecov Report

Merging #534 (bf73362) into trunk (3404fae) will decrease coverage by 0.07%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##            trunk     #534      +/-   ##
==========================================
- Coverage   70.90%   70.82%   -0.08%     
==========================================
  Files          47       47              
  Lines        3210     3215       +5     
==========================================
+ Hits         2276     2277       +1     
- Misses        934      938       +4     

Impacted Files	Coverage Δ
tests/alloc.rs	92.42% <33.33%> (-5.94%)	⬇️
src/enc/impls.rs	89.06% <100.00%> (ø)

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