There were conflicts in src/document/css/ because 0.12.GIT switched
to LIST_OF(struct css_selector) and 0.13.GIT switched to struct
css_selector_set. Resolved by using LIST_OF(struct css_selector)
inside struct css_selector_set.
Add ecmascript_interpreter.backend_nesting, increment it when
beginning to evaluate an expression, and decrement it when evaluation
finishes. Then assert that it is zero in ecmascript_put_interpreter.
This detects bug 957 and similar ones before they corrupt memory.
Replace the ECMAScript module initialisation and de-initialisation
routines that wrapped the SMJS and SEE module initialisation and
de-initialisation routines by having the module system call the SMJS
and SEE routines its own darned self.
