Span<T> goodness about formatwith HOT 2 OPEN

I've put together a PR with an initial implementation for your review (#20)

from formatwith.

@JeremySkinner I've finally got your PR merged, apologies it took so long!

It's still a work in progress, but I've overhauled the public FormatWith API to enable ReadOnlySpan<char> based scenarios. All of the existing FormatWith overloads are also now wrapping the lower level Span based overload.

The work is happening in the release-4.0 branch.

The main Span API usage looks like this:

HandlerAction handlerAction = static (key, format, result) =>
{
    if (key.Equals("KeyNumber1".AsSpan(), StringComparison.Ordinal))
    {
        result("A Nice Value".AsSpan());
        return true;
    }
    return false;
};

FallbackAction fallbackAction = static (result) =>
{
    result("A good fallback value".AsSpan());
};

string replacement = "abc {KeyNumber1} {DoesntExist}".AsSpan().FormatWith(handlerAction, MissingKeyBehaviour.ReplaceWithFallback, fallbackAction);
Assert.Equal("abc A Nice Value A good fallback value", replacement);

Currently there aren't any more "convenient" Span overloads (that take a dictionary or object lookup, for example), but I think they'll probably be needed. Also, the string returning overloads use a cache of ThreadLocal WeakReference'd StringBuilders which should provide a balance of performance and memory consumption.

There is also a lower level overload that accepts a callback delegate to write the result, and doesn't use StringBuilder at all:

public static void FormatWith(
           ReadOnlySpan<char> formatString,
           HandlerAction handlerAction,
           ResultAction destinationWriterAction,
           MissingKeyBehaviour missingKeyBehaviour,
           FallbackAction fallbackReplacementAction = null,
           char openBraceChar = '{',
           char closeBraceChar = '}')

Please let me know if you have any thoughts on the shape of these APIs, I'd love to get some feedback on their practicality and usability.

from formatwith.