Finish off removing context

[tcharding]

After #844 - some things we might want to do.

• Remove comment on alloc from manifest
• Fix all the stale docs
• Delete the global features (global-context and global-context-less-secure)
• Delete the old SECP256K1 object
• Delete all the foo_global functions (deprecated in Remove context from the API #844)
• Audit for any calls to Secp256k1::new in code, docs, or examples
• Release 0.32.0 and party

[apoelstra]

We also want to go through tests/api.rs and fix all the FIXMEs, and maybe expand those tests as well.

I think we'd then be in a position where if we could remove secp256k1-sys from the public API somehow, we could 1.0 this crate.

[apoelstra]

Certainly, we'd be able to go through #703 and start cherry-picking code that would let us constify nontrivial parts of the library.

[tcharding]

Shall we do v0.32.0 and then start on #703 as part of a push to 1.0? I did not have 1.0'ing this crate on my radar even.

[apoelstra]

Yep, sounds like a plan! #703 is gonna be tricky because it's a combination of fighting Rust/cargo and implementing crypto ops. So it's not going to be a fast process.

[tcharding]

Flagging that I think this issue and all discussion are covered by #858 and #859.