by defining the pointer contents to only be synchronized after explicit
sync points, makes it legal to have a fallback implementation based on
file operations while still supporting a handful of use cases for memory
mapping.
furthermore, it makes it legal for evented I/O implementations to use
evented file I/O for the sync points rather than memory mapping.
not yet done:
- implement checking the length when options.len is null
- some windows impl work
- some wasi impl work
- unit tests
- integration with compiler
Andrew Kelley