Add function to convert from explicit Type values to implicit Typed constraints

[RyanGlScott]

While studying the code in copilot-core recently, I noticed that there are many places that redundantly store Types at runtime. For instance, consider the definition of copilot-core's Array:

copilot/copilot-core/src/Copilot/Core/Type.hs

Lines 154 to 156 in 4e5fd9a

	Array::forallnt. ( KnownNatn
	, Typedt
	) =>Typet->Type (Arraynt)

This has both a Typed t constraint as well as a Type t value. We see something similar in the Spec data constructor, which has both a Typed a constraint and a Type a field:

copilot/copilot-core/src/Copilot/Core/Spec.hs

Lines 40 to 45 in 4e5fd9a

	dataStream=foralla. (Typeablea, Typeda) =>Stream
	{streamId::Id
	, streamBuffer:: [a]
	, streamExpr::Expra
	, streamExprType::Typea
	}

There's nothing incorrect about doing this, but it is wasteful. At runtime, Typed dictionaries already store a Type value (by way of the typeOf method), so one could get rid of the Type fields in favor of just the Typed constraints. Doing so would save some space every time GHC allocates a Type at runtime.

Alternatively, we could get rid of the Typed constraints in favor of just storing a Type field. In order to do this, however, we would need to be able to convert from Type (an explicit, value-level representation) to Typed (an implicit constraint). Currently, the copilot-core API does not offer a way to do this. I propose we add one. I'm thinking of a function with this type signature:

withTyped::Typea-> (Typeda=>r) ->r

This would be analogous to the Haskell standard library's withTypeable function. A simplistic way to implement withTyped would be:

withTyped::Typea-> (Typeda=>r) ->r
withTyped ty k =case ty ofBool-> k
Int8-> k
Int16-> k
Int32-> k
Int64-> k
Word8-> k
Word16-> k
Word32-> k
Word64-> k
Float-> k
Double-> k
Array{} -> k
Struct{} -> k

Alternatively, we could make withTyped a no-op at runtime if we are willing to use GHC's withDict primitive. (Doing so would require changing Typed to a single-method type class, so that is probably worth a separate discussion.)