build: refactor for simplicity

[davidhewitt]

This is a collection of a few small refactorings to build.rs:

• Made the error message slightly nicer when accidentally compiling on a Python version which is too old.
• Moved BuildFlags struct inside the InterpreterConfig, which I think is logically correct.
• The abi3_without_interpreter configuration now uses the same code path to emit cargo configuration as the rest of the script. This is motivated by changes in Add support for arbitrary arrays #1128 where I noticed the configure_const_generics() call was nearly missed out of the abi3_without_interpreter function, which would have led to confusing bugs.
• A few other minor updates to comments which had gotten very stale.

[davidhewitt]

CI failure looks like rust-lang/rust#84502 - I think we may have to live with coverage job being down for the next few days...