Fix FFI cache race condition

[jakelamberson]

The race condition exists in the following circumstance:

1. PyVEX process 1 parses ffi and opens a new cache file but has not written to it
2. PyVEX process 2 goes to parse ffi, sees the existing (empty) cache file, opens it, then fails when it attempts to read an empty file

This commit makes cache writing atomic, preventing the race condition.

Closes #418

[ltfish]

Thanks for the fix!