Ensure that the core files can be compiled noexcept

[axic]

The only exception currently seems to be the hex library and tests.

[axic]

@chfast is this still valid? https://github.com/ethereum/evmc/blob/master/include/evmc/hex.hpp still has a few functions without noexcept probably because fo .reserve().

[chfast]

You cannot mark all functions with noexcept. But you can compile without exception support -fno-exceptions.