Add variable-length shifts to asm and target encoders

[myreen]

The lack of variable-length shifts has been a blocker for a number of optimisations and improvements.

This issue is about adding variable-length shifts to asm and supporting them in all target encoders. This will help issues:

• Add word shifts to basis #274
• Add instructions for efficient Div and Mul by (powers of) 2 #246

I expect most of the work will be in updating the encoder proofs.

This issue can be closed once variable-length shift are available as allowed instructions in the lower end of the CakeML compiler.

[myreen]

For this issue, I suggest we do not change Shift in wordLang's exp type:

cakeml/compiler/backend/wordLangScript.sml

Lines 12 to 19 in d6e25b8

	Datatype:
	exp = Const ('a word)
	| Var num
	| Lookup store_name
	| Load exp
	| Op binop (exp list)
	| Shift shift exp num
	End

This issue is enough work without also breaking all the wordLang passes more than necessary.