DRTVWR-476: Add llsd::array() and llsd::map() variadic functions.

llsd::array(), as one might suspect, takes an arbitrary number of arguments of
arbitrary convertible types and returns an LLSD::Array constructed from those
elements. This supercedes the older LLSDArray class.

llsd::map() takes an even number of arguments paired as (LLSD::String,
arbitrary convertible type) and returns an LLSD::Map constructed from those
(key, value) pairs. This supercedes the older LLSDMap class.

These two functions not only have a simpler API -- arbitrary function
arguments rather than an (arg list)(arg list) sequence -- but also
specifically return a final LLSD object, rather than needing conversion to
LLSD from the LLSDArray or LLSDMap object.

Also support LLSD == LLSD and LLSD != LLSD comparisons, using llsd_equals()
with default exact-float-equality semantics.