Implement a derive macro

[jac3km4]

Another PR with some of my changes.
This adds derive macros for TryRead and TryWrite.

Some notes:

• supports borrowed types as long as the target of the macro has exactly one lifetime
• doesn't support enums, only structs
• by default it generates impls where the Ctx is set to Endian, this is the most ergonomic option, but it could be made more flexible with a macro attribute in the future
• there's a #[byte(read_ctx/write_ctx = ...)] field attribute that allows specifying custom context per field, the context expression can use values of preceding fields in the struct
• the resulting impl is a bit awkward to use sometimes because try_write is defined to take self by value, which necessitates cloning - this could be worked around by having the derive generate an impl for &Self, but that'll not work when you nest structs, I think the best option would be to re-define TryWrite to take &self instead of self
• this PR would be pretty easy to extend to derive the Measure trait I've added in another PR

Usage

#[derive(Debug, Clone, PartialEq, TryWrite, TryRead)]
struct Named<'a> {
    id: u32,
    timestamp: f64,
    #[byte(read_ctx = Str::Delimiter(0), write_ctx = NONE)]
    str: &'a str,
}