Add no_std support

[Kixiron]

It would be great to have no_std support for thiserror, and after a look over the codebase I don't think it would be very difficult, I'm just inexperienced with proc-macros. The only drawbacks (and thus make no_std compatibility require a feature flag) would be that std::error::Error and std::path are not core or alloc friendly. For the most part, migration would be easy as changing std imports to core or alloc, but for the aforementioned roadblocks I propose the following:

Add #![cfg_attr(feature = "no_std", no_std)] to the root of the thiserror crate to enable no_std for the no_std feature
Add a feature to both thiserror and thiserror-impl, possibly something like no_std, this would look about like this

# In thiserror
[features]
default = []
no_std = ["thiserror-impl/no_std"]

# In thiserror-impl
[features]
default = []
no_std = []

And then for problematic areas, like thiserror/src/aserror.rs and thiserror/src/display.rs, simply conditionally compile them when the no_std feature is not active using #[cfg(not(feature = "no_std))]
Lastly there'd be the normal changing std to core/alloc, but adding conditional compilation to places where aserror.rs and display.rs are used would also be needed

[dtolnay]

Closing in favor of #64 to keep the discussion in one place.

[Kixiron]

Oh, I'm sorry, I didn't see that PR