change in notations: @ for globals, % for locals, ^ for vpats

Syntax.var is now an abstract type, use Syntax.var_name to get the name
of a variable (without the identifying first character!).

This hides the internals of coreStar to clients, which do not have to
worry about following coreStar's conventions for variable names. Also,
looking at the first character is probably more efficient than matching
regular expressions, but maybe not given OCaml's awkward string
manipulation library.

It looks like global/local program variable distinction is not used
anywhere in coreStar yet, but it's now made more explicit (pvar split
into plvar and pgvar).

Tests painfully adapted to the new syntax.

Feel free to comment/revert.