TartanLlama's expected, optional & generator

This is the build2 port of the tl::expected, tl::optional and tl::generator projects:

• tl::expected : https://github.com/TartanLlama/expected
• tl::optional : https://github.com/TartanLlama/optional
• tl::generator : https://github.com/TartanLlama/generator
• build2: https://build2.org

Note for using this git repository as a package repository:

Because of the diverging version numbers of the packages stored here we decided to stop creating general version tag for releases like v1.0. We recommend users to directly use the packages version in the main branch or specific commits for now. For example, in your repositories.manifest:

:
role: prerequisite
location: https://github.com/build2-packaging/tl.git#main

See the Git Repositories section there for clarifications.