Move bignum.h to src/test

[laanwj]

After #4076, #3965 and #4048 bignum.h is only used for verifying scriptnum against.

[BitcoinPullTester]

Automatic sanity-testing: PASSED, see http://jenkins.bluematt.me/pull-tester/4def248b2b47e5440b8ed53b9dd12e8303601eb0 for binaries and test log.
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Contact BlueMatt on freenode if something looks broken.

[gavinandresen]

ACK

[laanwj]

Closing in favor of #4160 which does the same but also removes unused stuff from bignum.h.