Refactor JS runtime API

[dhil]

Patch #1087 refactors the JavaScript runtime. It is only an initial refactoring that compartmentalises and modularises code. The refactoring is particularly crude with regard to mailboxes, access points, and channels as the involved logic is rather convoluted.

We should redesign the interface of the core modules, e.g. _$Links, _$Proc, _$Session, _$Constants, _$Types.