Reorganize modules to restrict visibility

[pschafhalter]

Reorganizes mod.rs files so they follow the following format:

// Library imports (e.g. from std)

// Private submodules

// Crate-wide visible submodules

// Public submodules

// Private imports

// Crate-wide exports

// Public exports

// Functions, struct declarations, implementation, etc.

Also restricts visibility of submodules and other objects to reduce the amount of internal objects visible in the public documentation.