Clarify some of the Memo names used in Meta.Reflection.

I noticed when we merged a PR that the names of the various Memos in
Meta.Reflection were getting a bit confusing, so I've cleaned them up a
bit to 1) reduce confusion between what are now called
hasDeclaredFieldsMemo and declaredFieldsMemo and 2) to clarify what the
primaryConstructorArgumentsMemo actually contained.
Merge remote-tracking branch 'origin/master' into msf_issue_1519
Remove unused hasDeclaredField and its memo.
Merge pull request #1519 from lift/msf_issue_1519

Do some variable name cleanup in lift-json.

We had a memo named declaredFields and another named declaredFieldsMap,
that was added recently to prevent us from having to reflect on each retrieval of
the declared fields. These names were a bit confusing, so we changed them up.