This adds support for the nearly useless BTF_KIND_FLOAT, under the name
CTF_K_BTF_FLOAT. At the same time we fix up the ctf_add_encoding and
ctf_type_encoding machinery for the new API changes.
I expect this to change a bit: Ali Bahrami reckons I've oversimplified the
CTFv4 encoding representation and need to reintroduce at least a width.
New API:
ctf_id_t ctf_add_btf_float (ctf_dict_t *, uint32_t,
const char *, const ctf_encoding_t *);
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