Path hanger extern instead of crate declarations

[fmease]

Remove crate declarations (crate my-lovely-library) from the language and replace them by paths beginning with the path hanger external crates extern being a new keyword (extern.my-lovely-library).

Conceptually, we now have two roots in the module system: crate for the current crate being compiled(…) and crates for all the --linked library dependencies. For the purpose of documentation, let me inform you that before, we only had one – crate – and dependencies would be mounted into this single tree with duplicate crate declarations linking to the identical crate.

Motivations

• (original motivation) Allows macros, lowering passes, synthesizers of the type inference engine to generate self-contained expressions even if they need to refer to external crates, especially the standard library core!. Before, those would need to somehow inject a crate declaration somewhere close with a hygienic name. Horrendous! One example: Lowering sequence literals. Another one: Derive macros.
• Crate declarations are inconsistent with external module declarations in the regard how duplication is handled: External module declarations copy content, crate declarations _link`
• Allows to use bindings from an external crate directly with a single declaration (a use declaration) instead of two (a crate and a use declaration)
• Is more in line with the notion that external modules are added to your projects from the outside (whether that's a good thing or not) with --link or the crate manifest package.json5, rather than declared by the current crate (which might lead to the assumption that crate metadata is or should be in code like @(version ">=2.3.2") crate extension (whether that's a good thing or not))

Example

Currently

crate awesome
use awesome.Alpha

Beta: Type = Alpha Type

module sub =
    use crate.awesome

    gamma: … = awesome.stuff 0

module mu =
    module nu =
        crate awesome

        X: Type = awesome.f …

Then

use extern.awesome.Alpha

Beta: Type = Alpha Type

module sub =
    gamma: … = extern.awesome.stuff 0 ;; or `crate.awesome.stuff`

module mu =
    module nu =
        X: Type = extern.awesome.f … ;; or `crate.`…

[fmease]

Closed in #99.