Add symbolic profiling to Rosette.

Refactor the synthesize query to use assumption and assertion stacks.

Update all uses of the query to conform to the new syntax.
Change (vcgen e) and (vcgen-eval e) to return only the
assumptions and assertions generated during the evaluation of e,
without prepending the assumptions and assertions that were on
the stack before the form was evaluated.
Change (vcs e) accordingly to maintaing the eval/asserts semantics.

Merge pull request emina#29 from AlexKnauth/patch-2

use make-variable-like-transformer for fv

Add release notes for Rosette 2.1.

Update the Travis script.

Minor bug fixes.

Added objectives extraction that works only for optimizing constants …

…(not expressions).

Merge pull request emina#3 from jamesbornholt/persistent-smtlib

Make smtlib servers persistent.