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