Currently CHECK is used only in test and bench mark files except for one
usage in `ecmult_impl.h`.
We would like to move the definition of CHECK out of `util.h` so that
`util.h` no longer has a hard dependency on `stdio.h`.
Done in preparation for moving the definition of `CHECK` as part of an
effort to allow secp256k1 to be compiled to WASM as part of
`rust-secp256k1`.
This is a backport of secp256k1#1149