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