It now takes a context pointer (opaque data) that holds the buffers it
uses for the temporary strings. If the context pointer is null, a static
context is used (making those uses of va NOT thread-safe). Most calls to
va use the static context, but all such calls have been formatted
consistently so they are easy to find when it comes time to do a full
audit.
new va function: nva which returns a strduped buffer
expr.c options.c:
use nva instead of strdup (va (...
struct.c type.c:
make type encoding work properly for structs
