Generate stream items from expressions

[scothis]

Functions can return a stream, its items listed with @value, written as
the caller reads them. Streams of integers, floats or bool can continue with
items generated by an @expression, a closure in a subset of Rust, from the
items before them, the function's arguments, and their position.

• stream results: an empty stream from any function, a stream with items
  from an async function, written by a task lifted with a callback, nothing
  added to components without streams
• @expression closures over the previous items, starting from the @value
  items, or from the function's arguments, as items or as inputs read by
  name or through $init, with $call for the closure's parameters and $i for
  the position
• typed expressions: literals typed by context, as casts, wrapping
  operators (+% -% *% /% %% <<% >>% **% -%), ** for powers, methods,
  let and let mut bindings, assignments, if, while loops, and arrays
  or to_le_bytes/to_be_bytes generating several items at once
• a stream ends when its next item can't be represented, as Rust's checked
  arithmetic would fail, or runs until the reader closes it
• at most one @value and one @expression per doc comment
• a tree-sitter grammar for expressions, tested against the same corpus as
  the parser, and a specification for language servers, with a guide in
  docs/expressions.md
• math streams: odd, even, arithmetic, geometric, fibonacci, factorial,
  harmonic, figurate and prime, and new atomics and random components
• test-cli, composing test-cli-core with the data components, to call their
  functions from the command line with wasmtime run, reading streams as
  stdin asks