Add (optional) BIP62 rule checks

This implements rules 1 through 6 from BIP62, as a pre-requisite for BIP62 support in Toshi. These checks are all optional and are only performed when relevant options are specified in the call to verify_input_signature().

Looks like it's missing a +5 here.

From interpreter.cpp:   

```c++
    unsigned int nLenR = vchSig[3];
    if (5 + nLenR >= vchSig.size())
        return error("Non-canonical signature: S length misplaced");
```