Flattening and condensing repo

[jeremyfelder]

• condensed all readmes into a top level readme
• renamed cuda into icicle as it is the main library
• moved lib.cu into icicle as it is the entry point for the library
• updated build.rs to reflect restructure
• removed unnecessary wrapper and cargo workspace toml since we only have a single package
• added cargo.lock to the gitignore since we are building a library (see https://doc.rust-lang.org/cargo/faq.html#why-do-binaries-have-cargolock-in-version-control-but-not-libraries for more details)

[LeonHibnik]

@jeremyfelder add .gitignore