JSON (IR) backend needs fixes

[emil14]

First of all it's broken:

neva build -target=json e2e/interface_with_imports/main

failed to compile: json: unsupported type: map[ir.PortAddr]ir.PortAddr

Second, after we fix it we must add some e2e tests. And finally the name -target=json is a bit confusing, because among all program representation we emit intermediate one. Probably should be -target=ir. We can add options like (json would be the default format)

neva build -target=ir -format=json ...
neva build -target=ir -format=yaml ...