[LPeg] Switch matcher

["Soni L." <fakedme@...>]

Hello!

Would it be possible to add a "switch matcher" to LPeg?

It should be constructed as `pattern | {cases}`.

Here's how it works:

You match the pattern `pattern`. You use the result of that match to 
index into `{cases}`. `{cases}` is a key-value map from strings (match 
results) to patterns. If the key doesn't exist, the match fails, 
otherwise we attempt to match the indexed pattern (match the value we 
got from `{cases}`).

A default case can be done with `pattern - (pattern | {cases})`.

This is faster than using alternations: O(1) vs O(n).

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