libcdecl: Perform all parser allocations via cdecl__alloc_item.

libcdecl: Perform all parser allocations via cdecl__alloc_item.

The cdecl__alloc_item function internally does some basic initialization
and handles error reporting, so by using it consistently we can avoid
some of this stuff being repeated throughout the parser actions, which
gives a slight reduction in the size of the parser.

As this function allocates a fixed-sized union of all the parse tree
structures, this means a small amount of runtime memory may be wasted
for the smaller ones, but it should be less than 12 bytes per allocation.