Switch license from GPLv3 to LGPL

[Ph0enixKM]

Issue: #661



Is your feature request related to a problem? Please describe.
I have this case that I want to develop an app on AppStore for iOS which uses Amber compiler as a dependency. The problem is that Amber is licensed under GPL that does not permit using this compiler as a dependency without the entire project becoming licensed under GPL. Apps licensed as GPL are rejected by the Apple.

Describe the solution you'd like
I would like to propose switching to LGPL license which enables developers to use Amber as a dependency keeping a license they prefer. This is the only modification that LGPL introduces. Other restrictions of GPL will still apply.

All of current rust dependencies are under either MIT, Apache 2.0, MPL 2.0 which are compatible with LGPL.

To change the license, all contributors whose code is included in the current version of the project must explicitly state their approval in the comment under this pull request.

[Ph0enixKM]

I agree to relicense my contributions to Amber compiler under LGPL.

[hdwalters]

I agree to relicense my contributions to Amber compiler under LGPL.

[mks-h]

I agree to relicense my contributions to Amber compiler under LGPL.

[lens0021]

I agree to relicense my contributions to Amber compiler under LGPL.

[Mte90]

I don't accept the license change until #660 (comment)

[mks-h]

While reading up on Slint's licensing, I've found this interesting comment that argues it might not be possible for proprietary Rust applications to comply with LGPL as it uses static linking, while LGPL requires to make it possible to swap the LGPL component with a different one — what is usually possible with dynamic linking. Though according to this Reddit answer it is only a problem if the application's license is proprietary, as open source applications (under any OSS license) can be recompiled with a different component.