Allow users to clone functions

[StachuDotNet]

e.g. in this case, I'd like to generate many htmlTagX functions

[StachuDotNet]

An adjacent idea: generate Dark code from within Dark (so I can quickly generate a bunch of functions - either as fns that exist normally after the generation, or functions that are dynamically generated in ways I can change all at once later on.

[pbiggar]

Another reason for this is that developers want versioned functions so that they don't break existing code. We planned to allow locking handlers that are in use, so that devs can be sure to not break anything that's running - which would require copying the whole stack of functions.

[pbiggar]

Also, allow cloning handlers

[StachuDotNet]

merged into #5237