Configure options reorganization

* Introduce a new configuration option for assembly optimizations (which in the future will likely not just be for the field implementation), --with-asm=X, instead of it being part of --with-field=X.
* Make --without-asm and --without-bignum work.
* Move the knowledge of which options require what out of build-aux/m4/bitcoin_secp and into configure, leaving the functions there to just be checking for feature availability.
* Improve the summary printed at the end of configure a bit.