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).

src/types/from_sql.rs

@@ -4,6 +4,7 @@ use std::fmt;
 
 /// Enum listing possible errors from `FromSql` trait.
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum FromSqlError {
     /// Error when an SQLite value is requested, but the type of the result
     /// cannot be converted to the requested Rust type.

src/types/to_sql.rs

@@ -7,6 +7,7 @@ use std::borrow::Cow;
 /// `ToSqlOutput` represents the possible output types for implementors of the
 /// `ToSql` trait.
 #[derive(Clone, Debug, PartialEq)]
+#[non_exhaustive]
 pub enum ToSqlOutput<'a> {
     /// A borrowed SQLite-representable value.
     Borrowed(ValueRef<'a>),