std: propagate methods into global environment correctly. (issue #25)

* specs/string_ext_spec.yaml (keywords): List of methods that
should propagate into the global environment.
(it propagates keywords to the global environment): New
specification.
* src/std.lua.in: Propagate global methods correctly.
Fixes issue #25.
refactor: simplify specs/string_ext_spec.yaml.

* specs/string_ext_spec.yaml (context by name): Unroll a function.
(it adds extension apis to the global table): Remove debug code.