VM: add option to insert/override (custom) precompiles

[jochem-brouwer]

It would be nice, especially for custom chains, to insert or override precompiles in the VM. This should be done in the constructor of the VM.

[jochem-brouwer]

This might actually also go well with a slight refactor of the precompiles. We currently would not directly support different precompiles at the same address. These are all hardcoded.

[holgerd77]

If you identify useful refactorings along these examinations would be great as well, can very well also be breaking, then just do towards develop for the breaking releases.

Also, just to complement: not to forget the "remove" case - beside insert/override, but I guess this can be easily integrated in some coherent data structure, somehting like:

customPrecompiles = {
  address1 (not existing): function(), // insert
  address2 (existing): function(), // override
  address3 (exisiting): undefined // remove
}

? Just a very first shot.

[jochem-brouwer]

Yep, that makes sense, probably will follow the format I'm using in the custom VM opcodes (#1705).

[holgerd77]

Closed by #1813