This change removes the AbiSet from the AST, converting all usage to have just
one Abi value. The current scheme selects a relevant ABI given a list of ABIs
based on the target architecture and how relevant each ABI is to that
architecture.

Instead of this mildly complicated scheme, only one ABI will be allowed in abi
strings, and pseudo-abis will be created for special cases as necessary. For
example the "system" abi exists for stdcall on win32 and C on win64.

Closes #10049

…tri3

move `manual_c_str_literals` to complexity

IMO the suggestion to use `c""` literals over a hardcoded `\0` byte string literal has some fairly strong upsides (no need to manually null-terminate it and the compiler checks for intermediary null bytes) that this should just be enabled by default.

It's also written slightly conservatively to only emit a warning when we can be reasonably confident that the lint is *actually* applicable (that is, lint on e.g. `b"foo\0".as_ptr()` but not `b"foo\0"`. The latter looks like a c-string but its type is `&[u8; _]`, and if it's used in a context where a byte slice is needed then you have no nice way to convert to it from a `c"foo"` literal of type `&CStr`).

changelog: move [`manual_c_str_literals`] to complexity (now warn-by-default)