Log Message:
Rename rust-bin's PKGNAME to rust-bin. Add rust.mk for rust packages.

This allows rust-bin and rust to coexist in bulk builds (for testing, etc),
but the packages still may not be installed at the same time.

rust.mk as a solution for picking the correct rust variant was suggested
by gdt@. It is intended to be included directly by packages that do not
use cargo.mk, and indirectly by packages that do use cargo.mk.

rust.mk provides one user-settable variable:

RUST_TYPE
	as before, whether to bootstrap rust from source or use
	official binaries. may be "src" or "bin"

And two package-settable variables:

RUST_REQ
	the minimum version of Rust required by the package.
	defaults to "1.20.0"

RUST_RUNTIME
	whether Rust is a runtime dependency, may be "yes" or "no"