Support for ABI-specific integers

[mannprerak2]

Closes dart-lang/native#530.
This PR lays the groundwork for the generation of ABI-specific integers.

• Added 2 new configs - library-imports and type-map
• Removed config keys - size-map and typedef-map.
• type-map can target native types, typedefs, structs and unions.
• Update readme, changelog, version

[mannprerak2]

@dcharkes This PR currently does not generate or use any ABI-specific integers by default but it's left for the user to do by themselves via the given config. I think it'd better to wait till package:ffi has defined these ABI-specific integers before ffigen uses them by default.

[dcharkes]

This PR currently does not generate or use any ABI-specific integers by default but it's left for the user to do by themselves via the given config.

When there's a Dart dev release with it, I can make a dev release for package:ffi.

[mannprerak2]

When there's a Dart dev release with it, I can make a dev release for package:ffi.

I'm currently using the 2.16.0-dev.124 release which I guess supports Abi specific integers. So we can possibly hook up package:ffi in this PR if you want.

[dcharkes]

I'll make a pre-release once reviewed:

• ABI-specific integers ffi#128

[dcharkes]

Happy new year! 🧨 🎉

LGTM with comments addressed.

I think we should only map int8_t etc to dart:ffi types and target typedefs or AbiSpecificIntegers in package:ffi for all other types (including char, int, etc.)

[mannprerak2]

@dcharkes I've updated the code to use package:ffi everywhere, by using a git dependency. But this seems to have broken the tests on CI. Would update the PR to point to the dev version once it's released.

[dcharkes]

lgtm!

I'll try to get the package:ffi PR landed soon as well :)

FYI: https://dart-review.googlesource.com/c/sdk/+/228541 We're adding the C types to dart:ffi in Dart 2.17 (too late for 2.16) so that we can use them in @FfiNative annotations (which are used in places where we cannot import package:ffi). I'll update package:ffi to just export the dart:ffi definitions when Dart 2.17 comes around.

[dcharkes]

https://pub.dev/packages/ffi/versions/1.2.0-dev.0/changelog is here!

(Note I dropped ssize_t and off_t because they're only part of the posix extension not of C themselves. If we turn out to need these, we can make an extra PR to package:ffi to include some posix types.)

[dcharkes]

Thanks @mannprerak2! 🥇

https://pub.dev/packages/ffigen/versions/5.0.0-dev.0