Add source slices

[gavrilikhin-d]

SourceSlice may be helpful when:

1. Using slices into source instead of source+span. (nom uses this approach)
2. Using slice in error description

#[derive(Error, Diagnostic, Debug, Clone, PartialEq)]
#[error("return type mismatch: got \"{got}\", expected \"{expected}\"")]
pub struct ReturnTypeMismatch<'i> {
	/// Type of return value
	pub got: String,
        /// Span of returned value
        #[label("this has \"{got}\" type")]
        pub got_span: SourceSpan, 

	/// Expected type of return value
	pub expected: &'i str,
        #[label("return type specified here")]
        pub expected_span: SourceSpan,  // <- This may be inferred from `expected`
}

could be changed to

#[derive(Error, Diagnostic, Debug, Clone, PartialEq)]
#[error("return type mismatch: got \"{got}\", expected \"{expected}\"")]
pub struct ReturnTypeMismatch<'i> {
	/// Type of return value
	pub got: String,
        /// Span of returned value
        #[label("this has \"{got}\" type")]
        pub got_span: SourceSpan, 

	/// Expected type of return value
        // Maybe need to add something like: #[source_slice] 
        #[label("return type specified here")]  // <- assumes that this is subslice of source code
	pub expected: &'i str, 
}

[gavrilikhin-d]

@zkat what do you think of this idea? Would you accept a PR for this?