Generator: make `normalize_identifier` faster (#946)

`normalize_identifier` is called quite often in the generator, once for
every variable name or path element that is written.

This PR aims to speed up the function by

* using a per-length input string length replacement map
* binary searching the replacement map instead of a linear search

Diffent, but functionally equivalent implementations were compared:

```text
* linear search in one big map:      348.44 µs
* binary search in one big map:      334.46 µs
* linear search in a per-length map: 178.84 µs
* binary search in a per-length map: 154.54 µs
* perfect hashing:                   170.87 µs
```

The winner of this competition is "binary search in a per-length map".
It does not introduce new dependencies, but has the slight disadvantage
that it uses one instance of `unsafe` code. I deem this disadvantage
acceptable, though.

Nb. It was also tested if a variant that only stores the replaced string
would be faster. This "optimization" proved to be slower for all
implementations except "binary search in a per-length map", for which it
has the same runtime. Without a clear advantage to use the "optimized
version", I chose to use the more easy to read slice of tuples variant.

Obviously, for all measurements: YMMV.

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