Web updates

Web IDL 8d4e7ca67bcb

Clarifications to dictionary semantics This rewrites some of the text around dictionaries to clear up the confusion in #852, as well as generally try to smooth out the reading experience and definitions. Concrete changes: * Rewrote "dictionaries are always passed by value" to be more specific and clear. * Moved "dictionaries must not be used as the type of an attribute or constant" near the top of the dictionary section, commensurate with its importance. * Removed the terms "present" and "not present"; instead we can use Infra's "exists" for maps. * Fixed several cases where the overload resolution algorithm discussed arguments being "not present", even though that was only defined for dictionaries. Instead it now correctly talks about "missing". * Rewrote the discussion of required dictionary members and dictionary members with default values to make it clear how they contribute to the dictionary's entries. * Added a note to the IDL-to-ES conversion algorithm for dictionaries explicitly pointing out that default-value dictionary members will always be present, and thus always show up in the output. Closes #852. Closes #524.

Domenic Denicola