Add fallible APIs for `StringInterner`

[royaltm]

First I want to say that this is a pretty awesome project. It provides interning for strings only but that's exactly what I needed. Other interning crates are usually more generic, but this always comes with some strings attached (no pun intended).

My point being - this is the only decent interning crate that works for no_std that I was able to find.

That said, string-interner API is designed with an assumption most modern projects make: the memory is the last thing to go. (so is the Rust std/alloc btw. but that's another story and it's currently changing)

Unfortunately in embedded systems this is not the case, and RAM is usually one of the fastest-depleting resources.

You might think it's a bit insane trying to build a JIT MAL compiler for a little MCU board, but that's exactly what I'm doing 🤔.

My humble request is if you can add a fallible API calls to StringInterner so users can take some action on failure instead of just relying on a watchdog to restart the firmware on panic.

The failure being either running out of symbols or RAM whichever comes first.

For example:

fn try_get_or_intern<T: AsRef<str>>(&mut self, string: T) -> Result<<B as Backend>::Symbol, TryReserveError>;
fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError>;

#[derive(Debug, Clone, Copy)]
enum TryReserveError {
    NoMoreSymbols
    AllocationError
}

where try_reserve allows to change capacity ahead of time. This may also come with a complementary capacity method and a reserve method that panics.

I'm aware that this might be a big ask, involving substantial internal changes, but I'm ready to contribute myself if you are willing to go forward with this.

At least please let me know your thoughts. Thanks!