another attempt to move mem stat into base class

dependency cleanup - removed a lot of unecessary includes

improvements to predicate API
default rules encapsulated in LLInitParam
removed empty flag from viewer asset stats

added support for specifying predicates for xui and llsd serialization

added return value to serialize function to track if any values were written

sanitized param block code to accept isValid and isProvided at any point
added predicates to control serialization

finished most of conversion of llviewerassetstats
ported some param block fixes from viewer-chui
converted viewer asset stats to param block format

EXP-1482 FIX LLInitParam::Parser spam in secondlife.log file when entering a search term in location bar launching the search window
reviewed by Merov

reviewed by Leslie

added Flag as value type
moved Batch to BatchBlock
renamed Choice to ChoiceBlock
made merging of parameters for ValueParams consistent (fillFrom and overwriteFrom are inverses of each other now)
made iteration over Multiple<T> type params easier
initial schema param blocks

EXP-1063 FIX Destination Guide link in side panel does not open to destination guide specific search window
fixed param->LLSD serialization to not introduce empty strings

This way <foo/> or LLSD["foo"]; both define a default constructed value for the parameter named foo, useful in the Multiple<T> case

also made LLSD->param block parsing faster

ignore-dead-branch