libcdecl: Rework cdecl_explain output logic.

libcdecl: Rework cdecl_explain output logic.

Instead of adjusting the output pointer/length values after each
internal call, tweak all the functions in explain.c to take an
extra level of indirection so they can just directly adjust the
destination as they go.

A new helper, cdecl__emit, is provided to simplify the common pattern
of "print a single C string" followed by "advance pointer/length."