Rename brave_{wallet,news}/rust to rs

[antonok-edm]

After some discussion, we've settled on a convention for structuring our bundled Rust code:

• all Rust code is always located under an rs directory
• if there are multiple related crates, there are subdirectories for each: i.e. cxx for cxx bindings, lib for the core implementation, wasm for wasm bindings
• otherwise, the entire crate can go directly under rs without any subdirectories

brave_wallet and brave_news don't currently follow this convention; we should fix their paths. to improve consistency.

edit: Speedreader needs to be moved to use cxx, but in the meantime we can also rename its rust directory to rs instead.