make `parsers_sv2` compatible with `no_std`

[plebhash]

in order to build firmware that's compatible with rust-embedded, crates have to be compatible with no_std

we already had some contributions adapting some crates with this purpose:

• Protocols v2 no_std support #1394
• codec_sv2 with optional no_std #1346
• key-utils: make it no_std with a std default feature #1398
• subprotocol crates no_std support #1315
• Propose an implementation of noise_sv2 with optional no_std #1238
• enforce no_std for some protocol/v2 crates #1230
• make no_std into an optional feature on protocols crates #932

when these contributions happened, parsers was still a module of roles_logic_sv2.

recently, we made parsers_sv2 into a standalone crate.

this crate would be useful for firmware, since it would allow conversion of raw bytes into Rust-typed messages from the subprotocol crates (mining_sv2, job_declaration_sv2 and template_distribution_sv2, which are already no_std compatible)

[plebhash]

while we're working on this, it would be good to also do the following:

• evaluate the unimplemented! lines (if they can be cleaned up, or receive some meaningful implementation)
• crate an examples directory to showcase basic usage of the APIs

[Cyber-Lord]

I'll like to take on this or be fully part of the implementation.