Syntax extension for writing int63 literals.
This ppx allows users to write Int63.t values as literals, e.g. Int63.zero is 0J.
The J suffix stands for a "Jane Street integer", since Int63.t is defined in Jane Street's Base library; or alternatively, a JavaScript-compatible integer. More importantly, it was chosen to avoid confusion with digits, like I vs. 1, and confusion with existing well-known suffixes, like K for thousands.