`memoize_on_disk` can produce filenames that are too long

Since https://github.com/inducer/loopy/pull/710 (perhaps), `memoize_on_disk` produces quite long filenames, for example:
```
.../.cache/pytools/pdict-v4-loopy-memoize-cache-buffer_array-LoopyKeyBuilder.LoopyKeyBuilder-v0-2022.1-islpy2023.1.2-cgen2020.1-8158afdb18f100fb21b6cd86c6178e3c5cd19058-v1-py3.10.12.final.0
```

Unfortunately, some filesystems have quite short name restrictions. e.g. we had a bug report where this failed because the cache was on an encrypted filesystem where the max filename length is 144 characters (rather than the usual ext4 limit of 255).

Would it make sense to hash the (long) prefix as well as the contents?