Add a way for some profiles to inherit others

[sunshowers]

Cargo profiles have the inherits setting which lets some profiles inherit from others.

Nextest profiles should have the inherits setting as well. Steps to do:

• Add "inherits" to the [profile] data model. Treat an unspecified "inherits" as inheriting from the default profile.
• Check for cycles in the inherits graph (can use petgraph toposort for this, storing profile names in a GraphMap)
• Implement support for inherits -- in NextestProfile, store a chain of profiles rather than just one profile + default.

This is a really good first issue and will probably involve 1-2 days of work including tests -- please comment here if you'd like to work on it!