Syscall definitions

This is a work-in-progress definition of syscalls as FIDL.

This is not yet used by the build! Do not worry about updating it!

The location you're looking for is currently //zircon/system/public/zircon/syscalls.banjo.

Notes for interested in working on the transition

• Named xyz.test.fidl for now only to not require API review on gerrit, and to make it slightly more clear that they're not the real definitions yet.
• See tools/kazoo for the processor of these, and in particular the in-progress tests that compare the output of fidlc+kazoo to the current output of banjo+abigen.