Record variable declaration ranges

[turbolent]

Description

Record the ranges in which variable declarations are available, if position information is enabled (off by default).

This feature can be used to provide completion items in the language server.

Also, remove the unused Hash function for sema.Position and the dependency on fasthash.

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[janezpodhostnik]

LGTM

[janezpodhostnik]

Was this not used?

[turbolent]

It had been previously used / was previously needed when we used a persistent map/set library that used this function for the key. We replaced it and removed the dependency a while ago, and this was a left-over

[SupunS]

Looks good!