Experiment with context forking to increase thema concurrency

[sdboyer]

It's known that certain CUE API operations aren't safe for use in multiple goroutines simultaneously, within the same *cue.Context. (This issue itself needs larger-scale treatment than we're looking at here). In #67/ 45ac142 i introduced an internal mutex intended to help with this a bit. That was quite the sledgehammer, as it basically forces most operations within a thema.Runtime through a single mutex.

In looking through the CUE stdlib gocodec package , i noticed that for cases where the resultant cue.Values don't escape the scope of the call/modifications to the context need not be preserved, such as in Validate, they seem to aim at enabling greater concurrency by forking a separate context. We should explore something similar.

Aside, the gocodec package presents a Go interface to CUE that is functionally similar to Thema, but without the formal structures and invariants - it's worth plumbing for some other ideas, though it doesn't look like direct reuse is a good idea, if only because then we'd be doubling mutexes.