Add std implementation links to docs

[emesare]

It would be nice to have links to every rust std api used directly within the docs for the api.

For example:

rhai-fs/src/path.rs

Lines 14 to 22 in 896ea06

	/// Returns path to current working directory.
	///
	/// Throws an exception when:
	/// - The current working directory does not exist.
	/// - The process lacks the permissions to access the current working directory.
	#[rhai_fn(return_raw)]
	pub fn cwd() -> Result<std::path::PathBuf, Box<EvalAltResult>> {
	std::env::current_dir().map_err(|e| e.to_string().into())
	}

Becomes:

/// Returns path to current working directory. 
/// 
/// Throws an exception when: 
/// - The current working directory does not exist. 
/// - The process lacks the permissions to access the current working directory. 
///
/// See [std::env::current_dir](https://doc.rust-lang.org/std/env/fn.current_dir.html) for more information.
#[rhai_fn(return_raw)] 
pub fn cwd() -> Result<std::path::PathBuf, Box<EvalAltResult>> { 
   std::env::current_dir().map_err(|e| e.to_string().into()) 
} 

[emesare]

related: #9