Replaced nHashType with SigHashType in interpreter's SignatureHash and TransactionSignatureChecker functions

Summary: Continuing work on T61 to migrate to SigHashType.

Test Plan: Existing tests pass, which cover both valid and invalid base sig hash types. Note that although base type '0' has no valid use, it occurs in both the tests and in the current longest chain, so this implementation was tweaked to support it, yet still support strongly typed base types.

Reviewers: deadalnix, schancel, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D885