Reduce overhead of Swift runtime initialization for YYJSONDecoder

[mattt]

This PR attempts to resolve the significant cold start delay of YYJSONDecoder compared to Foundation JSONDecoder observed by @zshannon in #9.

Throwing spaghetti at the wall to see what sticks:

• We can reduce allocations by creating a _YYDecoderConfig class to share strategy configuration across all nested decoders and containers. This should help a lot for deeply nested JSON.
• If nothing else, having a YYJSONDecoder.warmup(for:) method gives API consumers an explicit way to amortize those startup costs on their own terms.

Other things we can try:

• Annotating with @inlinable + @usableFromInline to allow for cross-module inlining and avoiding witness table lookups
• Annotating hot paths with @inline(__always) to similar effect
• Annotating enums and structs with @frozen, which eliminates runtime metadata by fixing ABI layout at compile time

[mattt]

@zshannon When you get a chance, please give this branch a try alongside the 0.3.0 release of Replay. I'm very curious to see if fda9de6 made things better.