rust: use cargo new

[adsharma]

This allows directory mode transpilation to create a Cargo.toml
via cargo new --bin.

Any relative imports are transpiled to mod name instead of
extern create name.

Tested via:

head /tmp/foo/*.py
==> /tmp/foo/foo1.py <==
def foo1():
    return 42

==> /tmp/foo/main.py <==
from .foo1 import foo1

def main():
    print(foo1())

py2many --rust=1 /tmp/foo --outdir /tmp/rustfoo
cd /tmp/rustfoo
cargo run
42

Related to: #408