Feat/new mutant placing

[nicojs]

Implements a new mutant placing algorithm while also refactoring the instrumenter package.

🔢 New mutant placing algorithm

On the way down: 👇

• Treat the tree as immutable!
• Identify the nodes that can be used to place mutants on in the placementMap. I call these AST nodes "placement nodes".
  • Example: A node that is a Statement
• Generate the mutants on each node.
  • When a node generated mutants, do a short walk back up to the nearest node where the mutant can be placed, register them in the placement map
  • Call the new applied method on each Mutant, providing the placement node. This way the mutant will capture the AST with the single mutation from the placement node down.

On the way back up 👆

• If this node has mutants in the placementMap, place them in the AST.

🎨 Refactoring

• A Mutant now has an applied method. Note: passed tense, because it treats the input as immutable.
• Split each MutantPlacer's functionality in a canPlace method and a place method. The transformer uses canPlace to identity the placement nodes.
• Mutators no longer return an array of mutants. Instead, it returns an Iterable of replacement nodes.
• Mutators are now implemented as objects and pure functions (instead of classes). I think this programming paradigm fits better to the use case.
• Only the conditional expression mutator returned replacements for something other than the provided node. I've refactored that one mutator to be in line with the rest.
• The mutate functions now return an Iterable, since this allows us to use generator functions. You can simply yield mutants when needed. This cleans up the code nicely
• Split up babel-transformer into multiple methods
• The babel-transformer unit tests now use test doubles instead of sinon stubs. This makes for a more robust - black box - unit test experience.

Fixes #2968

[nicojs]

@Garethp feel free to review

[nicojs]

@Garethp I've merged it, but feel free to still review it later and add issues/ or a new PR later.