Make wasmparser compile for #![no_std]

[tomaka]

This make makes the wasmparser crate successfully compile for no-std platforms. The PR was very mechanical, but feel free to reject if this is out of scope of the repository.

I've added an std feature just because of a single impl std::error::Error for ... {}. My personal opinion would be to ditch this impl and the std feature entirely, but I went for the non-breaking option. Let me know if I should do this.

[alexcrichton]

I left a comment on a similar PR as well for Wasmtime, and I'd personally prefer to resolve it there first before figuring out whether or not to merge this.

[Robbepop]

@tomaka FYI I published a no_std compatible wasmparser crate here: https://crates.io/crates/wasmparser-nostd (in case you still need this)

It also switches out Hash{Map,Set} with BTree{Map,Set} internally so that the implementation is not attackable in no_std environment. Enable no_std by disabling default features (--no-default-features) as usual.

[Robbepop]

I think we can close this PR.

• It is outdated.
• We already have an open issue that informs people about no_std support in wasmparser et. al.
• The issue contains a link to the actual wasmparser-nostd fork that I maintain since a while.

[alexcrichton]

Sounds good.