Fix compatibility issue and other minor improvements

[AndreRoelofs]

First of all, thanks for such an amazing project. I use it every day!

My project started complaining about an incorrect compiler optimization. Namely

/home/andre/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/taurpc-0.5.2/src/lib.rs:305:13
    |
293 | /         if tauri::is_dev() {
294 | |             let result = export_types(
295 | |                 self.export_path,
296 | |                 self.args_map_json.clone(),
...   |
305 | |             result
    | |             ^^^^^^ expected `()`, found `Result<(), Error>`
306 | |         }
    | |_________- expected this to be `()`
    |
    = note: expected unit type `()`
                    found enum `std::result::Result<(), anyhow::Error>`
help: consider using `Result::expect` to unwrap the `std::result::Result<(), anyhow::Error>` value, panicking if the value is a `Result::Err`
    |
305 |             result.expect("REASON")
    |                   +++++++++++++++++

I looked at the repository and saw that one of the causes could be strict dependency requirements in the Cargo.toml. Version incompatibility would explain this weird error. I fixed it and confirmed that this solves the issue.

Here's a list of changes:

• Fix clippy comments
• Rename the rust field in Cargo.toml to rust-version and bump it to the version tauri currently requires to run
• Relax constraints on all dependencies except for specta (pre-release versions require explicit versions)
• Fix typos and other minor issues in documentation causing the tests to fail
• Bump taurpc version to 0.6.0

[MatsDK]

Hi, glad to hear you like the library. Thanks for making these changes! And feel free to let me know if you would like to see any other feature added to this library.