Writing a pure-Frothy library

Create a library with `lib.fr`, optional `lib.toml`, and no C.

A pure-Frothy library is just Frothy words. The required file is lib.fr. lib.toml is optional.

lib.toml:

name    = "servo"
version = "0.1.0"
boards  = ["host", "esp32_devkit_v1"]

lib.fr:

-- Attach a servo on a pin. Returns a handle to pass to the other words.
to servo.attach with pin [ pwm.open: pin, 50 ]

-- Move to an angle from 0 to 180 degrees.
to servo.write with servo, angle [ pwm.write: servo, map: angle, 0, 180, 250, 1250 ]

boards gates which boards may use the library. A library with no lib.toml is a pure-modules library that implicitly supports every board. Its name is its directory name.

If lib.toml is present, its name must equal the directory name.

Publish the directory to GitHub when you want other projects to consume it by URL and revision. frothy-servo is the real public example for that flow.