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.