Clarify where imports should be placed

[ftxqxd]

In most code, use imports are placed directly at the top of the file/module. However, sometimes I see them situated within a function. The Rust guidelines would be a great place to clarify where use imports should be placed. My recommendation would be to put all imports at the top of the module, except maybe for tests, where any imports specific to that function should be placed within that function only. (For example, if only one test uses std::collections::HashMap, the use statement should be placed within that function only.) Oh, and maybe main should also have any specific imports at the top of the function rather than the module.

[lilyball]

use imports that are expected to only ever used within a single function are reasonable candidates to put in the function. It's a judgement call.