Multiple problems in one repository

[KSAlpha]

Current implementation assumes that a repository contains one problem.

Support one-repository-multi-problem scenario.

• [x] separate repository and problem
• [x] soma_list.toml parsing
• [x] docker build support for multi-problem scenario

[Qwaz]

I think this should be included in 0.1.0 because implementing the feature is expected to introduce a lot of incompatibility to our API.

[KSAlpha]

Also consider moving some structs like Manifest and SolidManifest in soma::repo to soma::repo::problem or soma::problem.

EDIT: Moved to #92