Issue 4365: non-member unsmob<T> replaces T::unsmob and T::is_smob

Issue 4365: non-member unsmob<T> replaces T::unsmob and T::is_smob

Hide Smob_base<>::is_smob() to prevent the misleading situation of
D::is_smob(S) returning true when S represents an ancestor of D rather
than a D.  This issue was previously worked around by overriding
is_smob in several derived smob classes; that is no longer needed.

LY_ASSERT_SMOB() now returns a smob pointer.  This is not used yet,
but is available for future use to avoid unsmobbing twice.

Smob_base<>::smob_p() is hidden because there doesn't seem to be a
need for it to be public.

Files in lily/include were edited by hand.  Files in lily itself were
processed with the following sed script.

s/\([A-Za-z_]\+\)::derived_unsmob/unsmob<\1>/g
s/derived_unsmob *<\([A-Za-z_]\+\)>/unsmob<\1>/g
s/\([A-Za-z_]\+\)::unsmob/unsmob<\1>/g
s/\([A-Za-z_]\+\)::is_smob/unsmob<\1>/g
s/Input \*\([A-Za-z_]\+\) = unsmob /Input *\1 = unsmob<Input> /g
s/LY_ASSERT_SMOB (\([^)]*\))$/LY_ASSERT_SMOB (\1);/g