want deterministic letrec binding order to minimize diffs

[aryairani]

Describe and demonstrate the bug
We have seen letrec binding order swapped depending on changed dependency hashes, which is problematic for text-based diff tools.

Possible static orderings could be one of

• the order that the author specified, if preserved; currently not preserved
• alphabetical

For now, we might as well go with Alphabetical since they're not fixed anyway.

[aryairani]

It looks like letrecs are sorted by Symbol, and Symbols are sorted by freshness-id

unison/unison-core/src/Unison/Symbol.hs

Lines 28 to 29 in 90f6c78

	instance Ord Symbol where
	Symbol id1 name1 `compare` Symbol id2 name2 = (id1, name1) `compare` (id2, name2)

So we should try swapping that so that they're sorted by name (which is actually Unison.Var.Type) and hope that nothing to do with shadowing or scoping depends on Ord Symbol.