Turn on the new "ast" parser by default for execution.

This change replaces fish's execution model, and obviates much of
parser_t. Instead of parsing fish code into a sequence of
commands-arguments, this reifies syntactic constructs into a grammar,
builds a parse tree, and executes that. This provides a big
simplification and (sometimes) performance boost. fish while loops
become C++ while loops, etc.

There are some known regressions in error reporting, which ought to be
fixed in the soon-to-be-merged parser_cleanup branch. There's also
legitimate changes in edge cases. For example, `command builtin ...` now
executes a command called "builtin" instead of doing something else
weird. The most significant change is that syntactic elements must be
unexpected: for example, single quoting 'command' will now cause it to
not be recognized. This should be fixed soon.

Please open issues for any regressions you find!