Develop some combined infrastructure for command-line switches and crate attributes

[brson]

We are beginning to grow some crate attributes that affect compilation, such as #[crate_type], #[lint(no_ctypes)], #no_core. These are sprouting up in an add-hoc way and usually have corresponding command-line switches.

We should have some mechanism that makes it easy to define these and merge them into the session. Various things that are currently command line switches might ought to have crate attributes.

[graydon]

I'm still happy with the rule that "if it's required to make rustc foo.rs to work by default, it belongs in an attribute not a command-line option".

Following that rule -- which is "attribute heavy" in interpretation -- do we need this bug?

[brson]

No.