More kinds of static data for `(let)` bindings

[kiedtl]

There should be more possible syntaxes for a let binding value:

• (embed "path"): Embedded from a path relative to the current file.
• (fill <value> <count>): As name suggests. Filled at compile-time.

Examples:

(let font [Sprite8] (embed "../assets/atari8.uf2"))`
(let buf [U8 8] (fill 0xFF)) // Inferred

Tracker:

• (embed)
• (fill)