This revises and expands upon documentation for the standard library's
unsafe pointer types. This includes typed and raw pointers and buffers,
the MemoryLayout type, and some other top-level functions.
https://github.com/apple/swift-evolution/blob/master/proposals/0138-unsaferawbufferpointer.md
Unsafe[Mutable]RawBufferPointer is a non-owning view over a region of memory as
a Collection of bytes independent of the type of values held in that
memory. Each 8-bit byte in memory is viewed as a `UInt8` value.
Reads and writes on memory via `Unsafe[Mutable]RawBufferPointer` are untyped
operations. Accessing this Collection's bytes does not bind the
underlying memory to `UInt8`. The underlying memory must be bound
to some trivial type whenever it is accessed via a typed operation.
In addition to the `Collection` interface, the following methods from
`Unsafe[Mutable]RawPointer`'s interface to raw memory are
provided with debug mode bounds checks: `load(fromByteOffset:as:)`,
`storeBytes(of:toByteOffset:as:)`, and `copyBytes(from:count:)`.
This is only a view into memory and does not own the memory. Copying a value of
type `UnsafeMutableRawBufferPointer` does not copy the underlying
memory. Assigning an `Unsafe[Mutable]RawBufferPointer` into a value-based
collection, such as `[UInt8]` copies bytes out of memory. Assigning into a
subscript range of UnsafeMutableRawBufferPointer copies into memory.
