Restructure plpgsql's caching of 'simple' expression evaluation trees

Restructure plpgsql's caching of 'simple' expression evaluation trees
to be less dangerous, and often faster as well.  ExprState trees are
not kept across transaction boundaries; this eliminates problems with
resource leakage in failed transactions.  But by keeping them in a
per-transaction EState, we can safely arrange for a single ExprState
to be shared by all the expression evaluations done in a given plpgsql
function call.  (Formerly it seemed necessary to create and destroy an
ExprState for each exec_eval_simple_expr() call.)  This saves time in
any scenario where a plpgsql function executes more than one expression.
Seems to be about as fast as 7.3 for simple cases, and significantly
faster for functions that do a lot of calculations.