Move lambda type, dest_term function to kernel; add identical fn

dest_term is quite heavily used (it implements SML's best
approximation of a "view" for pattern-matching against terms), and its
implementation outside the kernel is needlessly inefficient.

Also extend signature with identical function, which correctly says
whether or not two terms are identical, including in their choice of
bound names. In the standard kernel, the possible presence of
explicit substitution terms means that this notion is not the same as
simple equality on terms.

Change src/bool to src/1 as a prelude to experimentation!