rust: add evcxr & rust-src toolchain component

[nothingmuch]

No description provided.

[lucperkins]

Does this work too?

(if builtins.pathExists ./rust-toolchain.toml then
  rust.fromRustupToolchainFile ./rust-toolchain.toml
else if builtins.pathExists ./rust-toolchain then
  rust.fromRustupToolchainFile ./rust-toolchain
else
  rust.stable.latest.default).override { extensions = [ "rust-src" ]; }

[nothingmuch]

on second thought, if a rust toolchain file is provided then it should be trusted WRT which components to use? (edit to clarify: maybe only rust.stable.latest.default.override { extensions = [ "rust-src" ] } should be used eliminating the need for let or parens, or more complex and seemingly still wrong but less wrong, append "rust-src" to rust-toolchain.toml component list?)