Mutate public to private methods

[deleugpn]

This PR:

• Adds new feature ...
• Covered by tests
• Doc PR Docs for Public to Private Mutator site#68

Fixes #498

This PR adds the ability to mutate from public straight to private methods.

[sanmai]

1. CI is failing because you need to run E2E tests and update fixtures for them (as now they see more mutations). The latter is typically done with ./tests/e2e_tests.
2. I'd make a private function makeClassMethod($visiblity): ClassMethod and avoid copy'n'pasting a bunch of code, what do you think?

[deleugpn]

Sorry, I did this PR at GitHub opening event in Amsterdam and forgot to check back the status. I'll look into the e2e test.
I swear I tried making a private method as you said and it was breaking the mutation.
I'll try again =D

[sanmai]

Please do. I'm here if you need a hand.

[miguelgf]

What improvement adds the yield here instead of the return?

[maks-rafalko]

this is a new feature (not yet released). It allows to return N mutations from one mutator. Note the second mutation on line 48 below.

Also, this is not the same as return [new ClassMethod(...), new ClassMethod(...)]. Such code replaces 1 Node with an array of Nodes, which is completely different behaviour.

[maks-rafalko]

Are you going to finish this PR @deleugpn or can we take over it?

[deleugpn]

Sorry, you can take it over. I hope to be back at some point.