feature: a bytecode format for units

[udoprog]

Units have been designed so that they can be serialized. This can be used in the future cache a compilation or distribute units in bytecode format for later execution.

This task is to design a preliminary format which will probably be subject to change before becoming stable.

A unit contains:

• A sequence of instructions, starting at 0x0.
• A hash-to-function map.
• A hash-to-type information map, which answers the question "what is this type hash?".
• Zero-based collection of static strings, byte arrays, and object keys.

To allow for checking that a context is compatible to execute a unit, the format should also include:

• The hashes of all functions used.
• The hashes of all types used.
• Optional debug information that can be shed to reduce size.

[udoprog]

Since 216daa3125838204531c9c95c2af8308ab2677df Unit now implements Serialize and Deserialize. And there's a prototype implementation in rune_cli which uses bincode to cache the compilation unit.

[shekohex]

we could also emit a .rnu.map a file that contains a map of the functions, locations that could be used for debugging .. I would prefer that to be a separate file instead of been embedded into the .rnu that would be similar to typescript source maps