mbedtls_ct_compiler_opaque() function

The same value that was passed in, such that the compiler cannot prove its value (even for calls of the form x = mbedtls_ct_compiler_opaque(1), x will be unknown).

This is mainly used in constructing mbedtls_ct_condition_t values and performing operations over them, to ensure that there is no way for the compiler to ever know anything about the value of an mbedtls_ct_condition_t.