Move Rust templates to wasm32-wasip1

[itowlson]

The Rust project has started the process of deprecating the wasm32-wasi target in favour of wasm32-wasip1. (There is also a wasm32-wasip2 but that isn't as mature I believe.) The new target was introduced in Rust 1.78. As of Rust 1.81, wasm32-wasi emits a deprecation notice; the target is slated to be removed in 1.84 (Jan 2025).

We therefore want to switch our Rust templates over to wasm32-wasip1, but to provide the least worst experience to users who have seen no reason to upgrade their perfectly good installation of Rust 1.73.

A possible way to do this is, at the same time we switch the template targets over, to add a MSRV of 1.78 to the templates' Cargo.toml files. This should ensure that people on older versions get guided to bump their Rust versions. However, I don't think upgrading to Rust 1.78 inherently installs the new target. So a user who just upgraded Rust would still get an error on the new target:

error[E0463]: can't find crate for `core`
  |
  = note: the `wasm32-wasip1` target may not be installed
  = help: consider downloading the target with `rustup target add wasm32-wasip1`

For more information about this error, try `rustc --explain E0463`.
error: could not compile `futures-core` (lib) due to 1 previous error

This isn't very lovely, but it does include the necessary guidance.

So I suggest that in time for the next minor release we:

• Bump Rust templates to wasm32-wasip1
• Bump Rust templates MSRV to 1.78
• Add docs calling out the name change and specifically including the error message

Are folks okay with this? Better ideas?