src/alloc.c: Remove all uses of `pure_alloc`

src/alloc.c: Remove all uses of `pure_alloc`

First step of removal of the purespace: stop using it.
The more delicate parts are the handling of 0-length strings and
vectors which we used to allocate in purespace but now need to be
allocated elsewhere, but the existing code makes us work harder to
allocate them in the normal way.

* src/alloc.c: Remove all uses of `pure_alloc`.
(init_strings): Alloc empty strings in the normal heap.
(init_vectors): Allocate the zero_vector in the normal heap.
(make_pure_string, make_pure_c_string, pure_cons): Rewrite to create
normal heap objects.
(find_string_data_in_pure, make_pure_float, make_pure_bignum)
(make_pure_vector, purecopy_hash_table): Delete functions.
(purecopy): Return without purecopying.