Convert into a workspace and import other UEFI crates

Convert this into a workspace containing all of the UEFI crates.

uefi/
├── Cargo.toml
├── crates/
│   ├── uefi/
│   │   └── Cargo.toml
│   ├── uefi_alloc/
│   │   └── Cargo.toml
│   └── uefi_std/
│       └── Cargo.toml
└── target/

The top-level Cargo.toml would be a virtual manifest. That is, it would only contain a workspace section and not a package section.

[workspace]
members = [
    "crates/*"
]