assembler: Import syntax

[wjmelements]

I want to be able to import one file into another. The import stack must be able to detect cycles.

I think it should work similar to the string syntax (#8), eg:

{
    string: "Hello World",
    hex: 0x3212,
    assembled: evm(src/ChildContract.evm),  // deployedBytecode, from `evm <>`
    constructed: init(src/ChildContract.evm),  // initcode, from `evm -c <>`
}

Then I would be able to build factories like

deploy:
CODECOPY(0, constructed, #constructed)
CREATE(GAS, 0, #constructed)

Currently I can workaround by hardcoding the output or building source with cat.