"is like" pattern matching operator

[edemaine]

Fixes #391

Most of this PR is code cleanup:

• getPatternConditions no longer requires a third conditions argument; it starts as an empty array that gets returned
• This lets the condition combination code get simplified a lot (also thanks to the recent . feature)
• New source/parser/ref.civet to house ref helpers
• New makeRefAssignment to ease creation of hoistDec and refAssignment
• New maybeRefAssignment = maybeRef + makeRefAssignment
• New flatJoin to ease combining expressions with && or ||
• More types (but still incomplete)

[edemaine]

I think we can add support for other positions in the future, but my thought was that everything else would end up being consumed by the pattern, so last position makes sense.

But we could have conditionals before is like. For example, x? is like ... could make sense as x? and x is like ..., similar to how x? < y? works now. Currently it matches the Boolean x? which isn't terrible useful. Similarly, x < y is like ... could make sense as x < y && y is like ....

UPDATE: Implemented!