Standardize Verifier names

[muraca]

I noticed that SigCheck and ThresholdMultiSignature follow two fundamentally different naming standards.
I think we should either push for the long extensive name, converting the SigCheck (where I think the word "Check" is superfluous) to something like SimpleSignature, or change the second one to some ugly contraption like ThreshMultiSig, which is still well understandable but less elegant.

@JoshOrndorff said:

I slightly prefer the long extensive names. [...] If you find it worth the effort afterwards, I trust your judgement to rename things, but I'm also fine leaving it as it is.