Make `head-meta` ^:const
Expose low-level fns: `freeze-to-stream!`, `thaw-from-stream!`
Move Reader fallback out of Freezable protocol to make protocol extensible
Add support for custom thaw readers
Add `custom-freezer` macro for easier Freezable extension
Switch to simpler `extend-freeze`, `extend-thaw` custom type API (ALPHA)
Perf: only set bindings when necessary
v2.1.0
Merge branch 'dev'