Mach.Port: Tidiness

[loffgren]

• Add leading underscores where necessary
• Freeze the Mach namespace
• Mark Mach namespace available in System 1.3.0
• Fix 80 column signatures

[karwa]

Nit: I believe this repo (like most apple repos) uses explicit access control for every symbol. There are public extensions in this file (lines 137, 284, 320 in the OG), which expose initialisers and such. They seem to be the first use of extension-wide access control in this repository. I believe the preferred approach is to annotate each public symbol individually, although it is best to check (pinging @lorentey).

Additionally, this is a generic type. Nothing in the implementation is @inlinable, so everything will go through unspecialised generic dispatch, which typically comes with a significant performance cost. If the lack of @inlinable is intentional, it may be worth a comment; otherwise, it may be worth annotating as appropriate.

[loffgren]

@swift-ci test

[loffgren]

@swift-ci test