Add BIP for CheckTemplateVerify

[JeremyRubin]

I believe now is an appropriate time to apply for a BIP number/Draft Status for OP_CTV. A few elements will require updating before the BIP moves out of draft (e.g., the activation dates) but activation dates are a separate discussion from the technical considerations which are the focus of this BIP.

[JeremyRubin]

BTW I looked into the travis error -- it seems that the linter is reading a code block and failing because a C++ lambda looks like a improperly formatted markdown link.

I am happy to add a revert-able patch as below:

bool filter(const CTxIn& c) {
    return c.scriptSig != CScript();
}
uint256 GetStandardTemplateHash(const CTransaction& tx, const uint256& outputs_hash, const uint256& sequences_hash,
                                    const uint32_t input_index) {
        bool skip_scriptSigs = std::find_if(tx.vin.begin(), tx.vin.end(), filter) == tx.vin.end();
        return skip_scriptSigs ? GetStandardTemplateHashEmptyScript(tx, outputs_hash, sequences_hash, input_index) :
            GetStandardTemplateHashWithScript(tx, outputs_hash, sequences_hash, GetScriptSigsSHA256(tx), input_index);
    }

so that you don't have to muck around with a broken linter/supressions, but I think it's better to fix the linter longer term.