Use efiapi for entry point

See merge request !5

Enable the ABI feature and use it for the entry point. This will set
calling conventions to what is specified by the UEFI spec.

It currently only sets x86-64 to `win64` and everything else to `C`.

Signed-off-by: Tim Crawford <crawfxrd@gmail.com>

Fix building on nightly

See merge request !3

Update dependencies and remove raw feature.

Signed-off-by: Tim Crawford <crawfxrd@gmail.com>

Use efi_main() instead of start() as name for entry point

See merge request !2

This is what the UEFI targets included in Rustc use for the entry point:
https://github.com/rust-lang/rust/blob/master/compiler/rustc_target/src/spec/uefi_msvc_base.rs

With this change, `uefi_std` can be used with the standard target
without passing a `-Z pre-link-arg=` argument to change the entry point
to `_start`.

Remove eh_unwind_resume lang item

See merge request !1

The eh_unwind_resume lang item has been removed [1], which prevents
using uefi_std with newer toolchains.

[1]: https://github.com/rust-lang/rust/pull/67502