[WIP] Cleanup `js::core::Context`, move to an extension model

[eddyashton]

Goal is for js::core::Context to be a pure interpreter (ie - we don't insert anything that QuickJS doesn't), and a vaguely readable interface. This PR works towards the latter by shuffling methods around, removing a few unused/unnecessary overloads, and the former by introducing an Extension pattern rather than populate_global_XXX methods on the Context.

A goal of this extension pattern is the removal of the messy globals object on Context - previously lots of inserted calls relied on the globals.tx object inserted to support the ccf.kv functions. Now they explicitly use the state in their own extension, including a Tx* where necessary. This should also simplify cleanup, as its a case of deleting extensions rather than invalidating global values.

[eddyashton]

TODO before merging:

• [x] Rename IExtension
• [x] Rename the extensions for less Ccf repetition
• [x] Manually verify what we expose in each execution context hasn't changed, see if that can be automated. (EDIT: Manually verified that we insert exactly the same globals, but with a bunch of hacks and manually string mangling. Will punt on automating this for now.