Add rust-toolchain file and replace cargo with rustup

- Avoid issue of compiling project with an older version of compiler and
getting error due to new API such as `Option::as_deref`.
- Will download and use binaries from rust-lang.org based on specified
version in rust-toolchain. This avoid the issue that compiler loads plugin
using different versions of glibc/ld than the ones used to produce the
plugin. See rust-lang/rust#58394

rust-toolchain

@@ -0,0 +1 @@
+1.45.0

shell.nix

@@ -9,9 +9,10 @@ pkgs.mkShell {
     git
     mypy
     neovim
-    cargo
-    rust-analyzer
+    rustup
+    rust-analyzer # For integration tests.
     tmux
+    vim # For manual tests.
     vim-vint
     (with python37Packages; [
       flake8