[Rust] Subtype checking only for reference types (WIP)

[chenyan-dfinity]

Implement #311

[github-actions[bot]]

Benchmark for 97d79e7

Click to view benchmark

Test	Base	PR	%
Blob/&str	264.7±83.26µs	241.9±74.44µs	-8.61%
Blob/ByteBuf	233.9±63.38µs	229.9±62.50µs	-1.71%
Blob/Bytes	145.5±35.83µs	141.7±36.49µs	-2.61%
Blob/String	334.9±87.23µs	340.0±92.39µs	+1.52%
Collections/vec (text, nat)	86.6±2.85ms	84.8±5.14ms	-2.08%
Collections/vec int	40.0±1.47ms	37.6±1.38ms	-6.00%
Collections/vec int64	29.5±1.59ms	25.9±1.68ms	-12.20%
Collections/vec nat8	20.9±0.70ms	20.7±0.97ms	-0.96%
option list/1024	3.0±0.12ms	2.5±0.14ms	-16.67%
profiles/1024	5.4±0.22ms	5.2±0.27ms	-3.70%
variant list/1024	2.9±0.12ms	2.8±0.20ms	-3.45%

[nomeata]

See the tests in https://github.com/dfinity/candid/pull/323/files if your haven't already

[chenyan-dfinity]

Close in favor of https://github.com/dfinity/candid/pull/395