Applies a conversion to the body of a paired abstraction.
If c is a conversion that maps a term t to the theorem |- t = t', then
the conversion PABS_CONV c maps abstractions of the form \p.t to theorems
of the form:
|- (\p.t) = (\p.t')
That is, ABS_CONV c "\p.t" applies p to the body of the paired
PABS_CONV c tm fails if tm is not a paired abstraction or if tm has the
form "\p.t" but the conversion c fails when applied to the term t.
The function returned by ABS_CONV p may also fail if the ML function
c:term->thm is not, in fact, a conversion (i.e. a function that maps a term
t to a theorem |- t = t').