tracers: fix err in 4byte, add some opcode analysis tools

[holiman]

This PR

• Fixes an error in the tracer 4byteTracer, which failed to check if a call destination was a precompile correctly, thus generating a lot of false positives.
• Adds a opcodeHistogram which just counts opcodes.
• Adds a bigram which counts bigrams
• Adds trigram which counts trigrams.

Although these are probably not useful in any general sense, having more examples that people can look at and base their own tracers from has a value too, thus I included the tracers in this PR.

[karalabe]

Nitpick, but could you give an example of the 3 tracer results here in the description? Since we suck at documentation, would be nice to have some minor trace at least here of what these do :)

[karalabe]

Please name all tracers with a _tracer suffix. Otherwise you have callTracer and bigrams and opcountTracer and trigrams. Lets keep all tracers uniform.

[karalabe]

Also, please rename opcodeHistorgram to unigram (if you introduced bigram and trigram too, lets keep them consistent).

[karalabe]

Alternatively maybe we could do ngram + introduce some parametrization?

[karalabe]

Fixed up the tracer names to unigramTracer, bigramTracer and trigramTracer.

[karalabe]

LGTM