Pre: move SQLValueType to core, because it's so central.

Yes, this isn't tidy... but in order to be really tidy we'd need to
split up db into parts that don't depend on a particular SQLite library.

core/src/lib.rs

@@ -76,6 +76,26 @@ impl TypedValue {
     }
 }
 
+// Put this here rather than in `db` simply because it's widely needed.
+pub trait SQLValueType {
+    fn value_type_tag(&self) -> i32;
+}
+
+impl SQLValueType for ValueType {
+    fn value_type_tag(&self) -> i32 {
+        match *self {
+            ValueType::Ref =>      0,
+            ValueType::Boolean =>  1,
+            ValueType::Instant =>  4,
+            // SQLite distinguishes integral from decimal types, allowing long and double to share a tag.
+            ValueType::Long =>     5,
+            ValueType::Double =>   5,
+            ValueType::String =>  10,
+            ValueType::Keyword => 13,
+        }
+    }
+}
+
 #[test]
 fn test_typed_value() {
     assert!(TypedValue::Boolean(false).is_congruent_with(None));

db/src/db.rs

@@ -331,25 +331,6 @@ pub fn ensure_current_version(conn: &mut rusqlite::Connection) -> Result<DB> {
     }
 }
 
-pub trait SQLValueType {
-    fn value_type_tag(&self) -> i32;
-}
-
-impl SQLValueType for ValueType {
-    fn value_type_tag(&self) -> i32 {
-        match *self {
-            ValueType::Ref =>      0,
-            ValueType::Boolean =>  1,
-            ValueType::Instant =>  4,
-            // SQLite distinguishes integral from decimal types, allowing long and double to share a tag.
-            ValueType::Long =>     5,
-            ValueType::Double =>   5,
-            ValueType::String =>  10,
-            ValueType::Keyword => 13,
-        }
-    }
-}
-
 pub trait TypedSQLValue {
     fn from_sql_value_pair(value: rusqlite::types::Value, value_type_tag: i32) -> Result<TypedValue>;
     fn to_sql_value_pair<'a>(&'a self) -> (ToSqlOutput<'a>, i32);

db/src/lib.rs

@@ -41,7 +41,6 @@ mod values;
 mod tx;
 
 pub use db::{
-    SQLValueType,
     TypedSQLValue,
     new_connection,
 };

query-projector/src/lib.rs

@@ -27,11 +27,11 @@ use rusqlite::{
 };
 
 use mentat_core::{
+    SQLValueType,
     TypedValue,
 };
 
 use mentat_db::{
-    SQLValueType,
     TypedSQLValue,
 };