Layering: track execution contexts' script/module

Fixes tc39#78. Introduces the concept of a Script Record as a counterpart to a Module Record, and adds [[HostDefined]] fields to both of them. Every execution environment now has ScriptOrModule component pointing back to its "creator" script or module:

- Top level script/module execution contexts point to the appropriate script/module.
- Function execution contexts for functions declared at top level point to that of the containing script/module.
- Function execution contexts for functions declared inside other functions point to that of the containing function, skipping built-in functions.
- Eval execution contexts point to that of their containing function (skipping built-in functions, including `eval` itself).
- Job execution contexts (which are largely a spec artifact) point to the script/module that originally enqueued the job.

In the course of doing this, a couple other substantial changes were introduced:

- Factored out ParseScript and ScriptEvaluation abstract operations, so that script parsing now better parallels module parsing.
- Added HostReportErrors for reporting both parsing errors and runtime errors, and wired it in to parsing and NextJob as appropriate.