Streamlining and minor cleanup of code in the PrettyNaming module.

Compilation and decompilation of operators is now memoized for custom
(non-built-in) operators to improve performance.
Lifted creation of lists (now arrays) in the IsInfixOperator function
so they aren't re-created (potentially) on each function call.
Simplification to the TryChopPropertyName function per @latkin's suggestion.
Optimized the recursive function which performs the decompilation within 'decompileCustomOpName' so it's tail recursive and avoids allocating substrings and some Option<_> instances.