Moving stdlib as repository

[Mte90]

The needs are specific, just to move the discussions in another place, so we can focus in this repo in the compiler itself (and builtins).

The flow:

• Wait for 0.3.5 release
• Merge all the pending PRs for stdlib stuff
• Create a new repo, https://github.com/amber-lang/amber-stdlib
• move them (to define if we can move also the git history or we don't care), remove from this repository (together with tests)
• Create in this repository the stdlib as gitmodule so we can block it to a specific commit, symlinked to the actual folders
• Update the CI and the readme

[CymDeveloppement]

@Mte90 do you think i can make a PR before moving stdlib ?
this is a PR for date function ( #325 )

[Mte90]

Yes just do everything now.
I am waiting the next release to move to a new repository so we have time.

Just don't forget to add the comments to the various functions.

[Mte90]

We have 2 folders:

That this repository need to include those 2 but the code organization is not fine for git submodules.

With a build.rs in the root that generate the simlinks we can achieve the symlink easily and in a transparent way for the user if need to compile it https://doc.rust-lang.org/cargo/reference/build-script-examples.html

[Mte90]

The other issue are tests but looking https://github.com/amber-lang/amber/blob/master/src/tests/stdlib.rs#L5 we don't need symlink