Allow usage of -preview=in

Allows building std.io when -preview=in is used. This adds a configuration for it, but can also be done via adding "dflags": ["-preview=in"] to your dependency specification.

Fix issue with dip1000

No changes to the code, just one attribute to fix a deprecation.

Fix issue with overriding Throwable.toString

This release is simply to get buildkite working with the latest compiler. It removes some @safe attributes on internals that were never safe to begin with.

Fix DIP1000 issue

Fix issue with buildkite and DIP1000. removes code previously intended to work around a bug that has since been fixed.

Fix issue with core.internal.string

See dlang/druntime#3169

Support Standard Handles

This release provides new accessors via std.io.driver package for stdout, stderr, and stdin to allow cross-platform ease of using these already-open standard handles.

There is also now a way to create io structures without having the structure automatically close the original file descriptor. This is useful for using io structures temporarily on already-open streams.

Fix CAS

Due to issue https://issues.dlang.org/show_bug.cgi?id=20359, io stopped building with DMD 2.089 and later. The issue still hasn't been fixed, but the workaround was simple enough and didn't change any code really. Now builds and passes tests with the latest compiler.

Meson Build

Thanks to @bioinfornatics, io will now build with meson. Also fixes a bug with ldc 1.18.0 and cas (thanks @kinke). Note, io does not build with 2.089.0, due to https://issues.dlang.org/show_bug.cgi?id=20359. When the bug is resolved, we will re-release if necessary.

Opaque Handles

This release fixes some issues with handle opacity, especially on Windows, and uses techniques that are more @safe-friendly on dmd 2.088.