Allow hook.Patch() to operate on ProgramSpec

From #5 :

Patching Program doesn't work because it's already loaded, using a ProgramSpec doesn't quite work either I think:
We can't remove the map from the CollectionSpec, so it will be created, duplicating the map created by hook.go. NewCollection() will overwrite the instructions we overwrote with RewriteMap to use the map it created, because RewriteMap preserves the instruction.Reference.

@lmb 's comments:

> We can't remove the map from the CollectionSpec, so it will be created, duplicating the map created by hook.go

I'm not super fussed by this, tbh. As long as the collection is closed it'll be closed afterwards. Something similar already happens for nested maps, etc.

> overwrite the instructions we overwrote with RewriteMap to use the map it created

That's an interesting case. Should the collection loader in that case refrain from patching the map twice? We could look at ins.Constant = 0.

Start by tracing hook.Patch() and hook.go alongside ProgramSpec, CollectionSpec, NewCollection(), and RewriteMap(). Determine how map creation and instruction references interact during loading; done means hook.Patch() can operate on ProgramSpec without its rewritten instructions being overwritten or maps being patched inconsistently.

Written by the indexing model from the issue text.