implement field visibility

[metagn]

Code is a bit ugly, can be cleaned up if necessary

Currently checks for the line info of the field access to compare modules. An alternative might be to track the current instantiated routine symbol in a stack which would include templates

Imported modules don't seem to compile to C yet so had to disable testing the working cases

The type conversion in:

template getPrivateTempl*[T](x: Generic[T]): T =
  T(x.private)

is a workaround, the type of x.private is the T in the original definition of Generic. This only errors for templates because other routines use commonType since #320, which matches anything to typevar expected types. Maybe x.private can substitute the type of private according to the InvokeT params of the type of x, this would have to be done for inherited types too.

[Araq]

Wait ... what? Why?

[metagn]

Exporting a symbol is only allowed if the current scope has kind ToplevelScope, so this is propagated from object declarations in toplevel scopes to the scope of their fields so that the fields can be exported. The only other use of scope kinds seems to be for whether or not to make global or local syms in identToSym which already unconditionally makes global syms for field symbols, so I thought it was sufficient to change it.

Generics in general also open a new scope though (the other place where this is done) which is a bit more murky, this is supposed to work:

type Foo[T] = object
  field*: T

But this would compile even though it doesn't add the symbols to the top level scope:

type Foo[T] = (; const field* = 123; T)

Maybe a ToplevelFieldScope kind instead?

[Araq]

Ok, but can be done later.

[Araq]

Code is a bit ugly, can be cleaned up if necessary

We rarely if ever find the time to clean up things so it's better to clean it up right now. That said, I don't find it particularly ugly.