rewriter: wrap AbiSignature in FnSignature that also stores API info (arg/return types)

[kkysen]

This wraps AbiSignature in a more generic FnSignature that also contains API info as well as the existing ABI info. The API info is stored in ApiSignature, which contains Params for the args and return. Each Param contains the parameter name (if a parameter), the type name, the canonical type name, and a type ID based on the canonical name. For now, the type ID is counted by interning the canonical types, so it's unique within a single ia2-rewriter invocation.

Next, we can use this API info to both define C functions (if we want to) and call into the type-registry library (#493) with type IDs for the arguments and return type.

[randomPoison]

There seem to be a lot of unrelated formatting changes in here. Are you making these changes manually or is this formatting style being enforced by clang tidy or something? Either way it'd be better to split the formatting changes into their own PR, rather than lumping them in here with other functional changes.

[kkysen]

There seem to be a lot of unrelated formatting changes in here. Are you making these changes manually or is this formatting style being enforced by clang tidy or something? Either way it'd be better to split the formatting changes into their own PR, rather than lumping them in here with other functional changes.

There are a lot of files that aren't formatted (nothing in CI checks this), but it's convenient to autoformat when I'm editing, so I formatted these files that I'm editing here. I did split them into separate commits, but I wasn't sure if a whole PR is worth it. But I can do that; it's pretty trivial.

[randomPoison]

I did split them into separate commits, but I wasn't sure if a whole PR is worth it. But I can do that; it's pretty trivial.

Nah, don't worry about it here. I would just try to do separate PRs for any non-trivial formatting in the future.

[randomPoison]

Looks good to me, though I'm not familiar enough with the rewriter to know if this is the best solution. I defer to @ayrtonm and @fw-immunant to judge that.