Correct Smob names in "Wrong type" error messages

Move the code that was used to demangle the name of base Smobs into a template function, allowing it to be used for derived classes too.

In LY_ASSERT_SMOB, stop relying on a manually maintained predicate-to-typename table. Instead, use the new function to get the Smob name.

Tested manually with this input:

\version "2.23.5"
#(ly:context-name #f)
#(ly:item-get-column #f)

Closes #6206 (closed).