Add #[non_exhaustive] to enums that might get new variants. (#673)

This just using them in patterns without a catchall. I left things alone
that seem very unlikely to change (`Value`, `ValueRef`, `DatabaseName`,
etc...). This might help reduce the number of breaking changes we need
(rusqlite is still pre-1.0 so it doesn't really matter that much, but
breaking changes complicate the story around when we can cut releases).

libsqlite3-sys/src/error.rs

@@ -4,6 +4,7 @@ use std::os::raw::c_int;
 
 /// Error Codes
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[non_exhaustive]
 pub enum ErrorCode {
     /// Internal logic error in SQLite
     InternalMalfunction,

libsqlite3-sys/src/lib.rs

@@ -17,6 +17,7 @@ pub fn SQLITE_TRANSIENT() -> sqlite3_destructor_type {
 
 /// Run-Time Limit Categories
 #[repr(i32)]
+#[non_exhaustive]
 pub enum Limit {
     /// The maximum size of any string or BLOB or table row, in bytes.
     SQLITE_LIMIT_LENGTH = SQLITE_LIMIT_LENGTH,