Move signature cache from CKey::Verify to CheckSig in script.cpp

Move signature cache from CKey::Verify to CheckSig in script.cpp

More than doubles the speed of verifying already-cached signatures
that use compressed pubkeys:
Before: ~200 microseconds
After:  ~80 microseconds
(no caching at all: ~3,300 microseconds per signature)

Also encapsulates the signature cache code in a class
and fixes a signed/unsigned comparison warning.