Beta-reduces all the beta-redexes in the conclusion of a goal.
When applied to a goal A ?- t, the tactic BETA_TAC produces a new goal
which results from beta-reducing all beta-redexes, at any depth, in t.
Variables are renamed where necessary to avoid free variable capture.
A ?- ...((\x. s1) s2)...
A ?- ...(s1[s2/x])...
Never fails, but will have no effect if there are no beta-redexes.
Beta-reduction, and indeed, generalized beta reduction (GEN_BETA_CONV) are
already among the basic rewrites, so happen anyway simply on REWRITE_TAC.
But occasionally it is convenient to be able to invoke them separately.