[WIP] Replace ConstChoice with ctutils::Choice

Depends on RustCrypto/utils#1266

Extracts all of the `const fn` constructor and predication/selection
methods used by `crypto-bigint` into the `ctutils` crate, allowing the
`ConstChoice` type to be removed and replaced by `ctutils::Choice`.

Author: tarcieri

Cargo.lock

@@ -17,6 +17,7 @@ edition = "2024"
 rust-version = "1.85"
 
 [dependencies]
+ctutils = { version = "0.1", features = ["subtle"] }
 subtle = { version = "2.6", default-features = false }
 
 # optional dependencies
@@ -85,3 +86,7 @@ harness = false
 
 [profile.dev]
 opt-level = 2
+
+[patch.crates-io.ctutils]
+git = "https://github.com/RustCrypto/utils"
+branch = "ctutils/crypto-bigint-methods"