Scanner: the bencode grammar over buffers the caller feeds, with
error.BufferUnderrun when a feed runs out and a cursor for slicing a
container back out of the input.Reader: the scanner driven from any std.Io.Reader, with borrowed byte
strings and nextAlloc when the caller needs to own one.decode and decodeLeaky: a whole value as a Value tree, with Document
owning the arena.Dict: stdx's byte-ordered map, so keys are always in the order bencode
requires.Writer and Value.format: write a tree, or the pieces one at a time.
