Fractional permissions for mutable and immutable borrowing

[starsandspirals]

Introduces the extensions from the fractional uniqueness paper into Granule. Currently includes:

• The additional 'borrow' modality, graded by a permission
• Fractional grades as one instance of permissions, with algebraic operations and SMT constraints
• withBorrow, split, join, push, pull primitives for mutable and immutable borrows, as in the paper
• Well-typed interface for polymorphic references

Remaining tasks before this can be merged:

• Existentially typed identifiers for arrays and references
• Runtime behaviour for references (should be represented via Haskell IORefs managed via unsafePerformIO as with mutable arrays, pointing to Granule AST values)
• Star alias for a 'unique' permission, additional to fractions
• Derivable push and pull for arbitrary data types (let's call this a stretch goal, but it would be great to get there, and we do have an example based on this in the paper - can follow exactly the same schema as deriving for the box modality)
• More descriptive error messages when working with borrows/fractions (minor)
• More practical examples using arrays and references (minor)

[dorchard]

@starsandspirals - what's the status of this? I think it's pretty much ready to merge?

[dorchard]

Bump on @starsandspirals - I believe we can merge this right?

[dorchard]

I resolved the conflicts.