docs: formatting comments with double slash (apple#1881)

Motivation:

Triple slashes should be for exportable documentation (like functions declarations).

Modification:

Replaced `///` with `//`.

Result:

Better looking code comments.

Co-authored-by: Antoine Cœur <[email protected]>

Tests should tolerate cpusets. (apple#1853)

Motivation:

Our current CPU pinning tests do not expect to be pinned to cores that
do not include core 0. That's unnecessary.

Modifications:

- Expect to pin to the first core in our cpuset.

Result:

Tests run inside cpusets.

Implement reserveCapacity for CircularBuffer. (apple#1819)

Motivation:

While spelunking through the code I noticed we called
CircularBuffer.reserveCapacity, but never actually implemented it. This
means it never did anything. It should do something.

Modifications:

- Implemented reserveCapacity.

Result:

reserveCapacity does something now.

Make HTTP Parser import implementationOnly (apple#1776)

Motivation:

Non-implementation-only imports in Swift have an annoying tendency to
put C header files into all downstream build paths. The result is that
leaf packages that include any C header files run a very high risk of
colliding with declarations in the inner header files. In this case,
we've had users bump into issues with the HTTP_STATUS_* enum cases
already.

Modifications:

- Make CNIOHTTPParser import implementationOnly where possible.

Result:

Fewer risks of colliding symbols.

update code of conduct to version 1.4 (apple#1753)

Make all imports of CNIOLinux conditional on OS (apple#1704)

Motivation:

Some users are including NIO in Xcode workspaces
rather than using SPM.  When this is done, if all
imports of CNIOLinux are conditional they can
remove this project from their workspace.

Modifications:

Make all imports of CNIOLinux conditional on Linux,
Android or FreeBSD

Result:

No user visible change.

Remove use of removeAll(keepingCapacity:). (apple#1699)

Motivation:

As outlined in https://bugs.swift.org/browse/SR-13923,
removeAll(keepingCapacity:) on Array has particularly negative
performance when that Array is not uniquely referenced. In this case, in
EmbeddedChannel, we _arrange_ to multiply reference it. This makes it
swamp our HTTP/2 microbenchmarks, spending more cycles copying this
buffer around than doing anything else.

Modifications:

- Just allocate a new buffer instead.

Result:

Much less copying.

Conform TimeAmount to AdditiveArithmetic (apple#1691)

Conforms TimeAmount to AdditiveArithmetic.

Motivation:
TimeAmount does not support -=, +=. Sometimes it is useful to manipulate time amounts when building up a delay and if that iteration fails, we would want to delay += .milliseconds(5) to add 5 milliseconds to our delay and try again.

Modifications:
Conformed TimeAmount to AdditiveArithmetic: added a static zero property and required operators.

Result:
TimeAmount conforms to AdditiveArithmetic.

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

Minor cleanups of Windows compatability code. (apple#1663)

Motivation:

Cleanup some noise that came in with some previous Windows compat
patches.

Modifications:

- Move INVALID_SOCKET to NIOBSDSocket, and refer to it by a better name
  there.
- Comment in an empty block to indicate that it's supposed to be empty.

Result:

Bit cleaner code.

Note that certain NIOFileHandle.init overloads are blocking (apple#1630)

This was noted in review  of swift-server/async-http-client#275, but I don't think that this was clearly stated in the documentation.