Move non-trivial uint256.h methods to uint256.cpp #4437

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Tested ACK

It's unfortunate that util depends on uint256, otherwise this could go to common lib instead of util lib.

@sipa Maybe after #4400 it could. AFAIK bitcoin-cli doesn't use any random functionality.

@sipa @laanwj Trivial to fix. Current master only shows util.{h,cpp} needing uint256 in a single function: GetRandHash() "Rand" stuff should arguably move to crypto/ anyway. Problem solved.

Also, #4332 adds the core_{read,write} modules as a home for converting from hex/json-to-core data structure (and the reverse direction). Some hex parsing can be moved there, or to a separate hex module.

Shrink util. :)

@jgarzik yes, see #4400. It moves the Rand stuff to a separate file. We decided not to move it to crypto because it still relies on OpenSSL but that could be done at a later time.

Very ACK on shrinking util.