Readd module _NIOConcurrency to not break adopters (apple#1970)

Work around Xcode 13 GM SDK issues. (apple#1956) (apple#1968)

Motivation:

Xcode 13 GM shipped with a Swift overlay for libsystem in macOS that
marked free's first argument as non-nullable. This leads to an awkward
breakage for us, because we're trying to hold a reference to free as a
function pointer, and to do that we had an explicit type annotation.

We'd like to keep NIO compiling in Xcode 13 GM.

Modifications:

- Defined the free function as a thunk that passes through to the
  underlying OS free call, but takes its first argument as non-nullable.

Result:

Should compile on the Xcode 13 GM again.

(cherry picked from commit fb48bdd)

Work around Xcode 13 GM SDK issues. (apple#1964)

Motivation:

This is a backport of apple#1956.

Xcode 13 GM shipped with a Swift overlay for libsystem in macOS that
marked free's first argument as non-nullable. This leads to an awkward
breakage for us, because we're trying to hold a reference to free as a
function pointer, and to do that we had an explicit type annotation.

We'd like to keep NIO compiling in Xcode 13 GM.

Modifications:

- Defined the free function as a thunk that passes through to the
  underlying OS free call, but takes its first argument as non-nullable.

Result:

Should compile on the Xcode 13 GM again.

Co-authored-by: Cory Benfield <[email protected]>

Work around Xcode 13 GM SDK issues. (apple#1956)

Motivation:

Xcode 13 GM shipped with a Swift overlay for libsystem in macOS that
marked free's first argument as non-nullable. This leads to an awkward
breakage for us, because we're trying to hold a reference to free as a
function pointer, and to do that we had an explicit type annotation.

We'd like to keep NIO compiling in Xcode 13 GM.

Modifications:

- Defined the free function as a thunk that passes through to the
  underlying OS free call, but takes its first argument as non-nullable.

Result:

Should compile on the Xcode 13 GM again.

Clarify the support status of NIO1 (apple#1949)

Motivation:

It's currently unclear that SwiftNIO1 is end of life.

Modifications:

Update the README to set a deadline on patches to NIO1

Result:

Everyone knows where the core team stands on supporting NIO1

Co-authored-by: George Barnett <[email protected]>

Add missing implementation of subscript(_ bounds:) to StaticString (a…

…pple#1947)

Motivation:

This is a backport of apple#1893 for SwiftNIO1.

Modifications:

Add an implementation of subscript(_ bounds:) to StaticString.

Result:

* StaticString can be sliced without crashing, though in O(n) time.
* The package should build with the July 8 2021 snapshot of Swift.

[Concurrency] `completeWithTask` expects to receive a `@Sendable` clo…

…sure (apple#1943)

Mention NIOPosix in README. (apple#1940)

add setters to complete implementations of MutableCollection (apple#1925

) (apple#1926)

- for `CircularBuffer` and `MarkedCircularBuffer`.
- with tests.

(cherry picked from commit 5b1f0c6)

Co-authored-by: Guillaume Lessard <[email protected]>

remove NIO1 API shims (apple#1897)

Motivation:

The lastest NIO versions require Swift 5.2+ to compile. Given that
hopefully nobody ever created a NIO1 application for Swift 5.2+, I'd say
it's about time to remove the NIO1 API shims (and related docs).

Modification:

- remove the NIO1 API shims
- remove the migration docs

Result:

Less code.