Hello,

this patch adds preshed[1] under the name 'python-preshed'.

The package depends on:
- 'python-murmurhash' (this package was recently merged to Guix.)
- 'python-cymem' (this package is awaiting the merge.)

'python-preshed' is the one of the dependencies of spaCy -- an NLP
library for Python.[2]

- Artyom

References:
1. "Cython hash tables that assume keys are pre-hashed"
   https://github.com/explosion/preshed
2. https://spacy.io/