Port and adopt main written in Rust

We don't change anything about compilation-setup, we just immediately jump to
Rust, making the eventual final swap to a Rust entrypoint very easy.

There are some string-usage and format-string differences that are generally
quite messy.

fish-rust/build.rs

@@ -62,6 +62,7 @@ fn main() {
         "fish-rust/src/fds.rs",
         "fish-rust/src/ffi_init.rs",
         "fish-rust/src/ffi_tests.rs",
+        "fish-rust/src/fish.rs",
         "fish-rust/src/fish_indent.rs",
         "fish-rust/src/function.rs",
         "fish-rust/src/future_feature_flags.rs",