Creating a secp context in `to_public_key` is expensive when deriving many elements

[RCasatta]

rust-miniscript/src/descriptor/key.rs

Line 1321 in 7ba28c0

let secp = Secp256k1::verification_only();

I am generating a lot of script pubkeys from a descriptor and creating the context every time is pretty expensive (about 20 microseconds each)

Clearly is convenient not passing the secp context in the API in most cases that are not requiring generating a lot of scripts/address so I am not sure how to improve this but I wanted to point this out

[apoelstra]

Yeah, this is a problem in several places in rust-miniscript. The next version of rust-secp should eliminate the context object praameters in favor of using thread-locals. See rust-bitcoin/rust-secp256k1#806

Although, I suspect that creating public keys will be even slower because of the rerandomization. We may want to offer a knob to turn it off at runtime if you're really performance-sensitive.