In Operator

Lex `in` as IN_OPERATOR instead of IDENTIFIER.
In Operation

`in` is a binary operation that works on `twoOperation`,
`additionOperation`, `multiplicationOperation`, `hatOperation`,
`unaryOperation`, `atOperation`, `keywordIdentifier`, `value`, or
`emptyParentheses`.
Test In Operation precedence

Test that lower precedence operations (those with choices before
inOperation in expression) wrap inOperations.
Highlight in operator as Operation Sign
Add In operator to README

Add to list of operators.
Land #52, In Operator
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