Schnorr signatures can now be passed to VerifySignature.

A function IsSchnorrSignature was also added to sigencoding.h, since in a
future revision this will also be called from CheckRawSignatureEncoding.

Depends on D2373 and D2348

(replaces D2355)

new test module (sigcache_tests.cpp) added, to make sure that
Schnorr/ECDSA cache collisions are not occurring. When I tried adding
SCRIPT_ENABLE_SCHNORR into the cache's INVARIANT_FLAGS, then this test fails
as expected.