core: Make Value::Text reference counted

Avoids lots of memory copies as values get passed through the different
layers.

core/sqlite3_ondisk.rs

@@ -403,7 +403,7 @@ pub fn read_value(buf: &[u8], serial_type: &SerialType) -> Result<(Value, usize)
             }
             let bytes = buf[0..n].to_vec();
             let value = unsafe { String::from_utf8_unchecked(bytes) };
-            Ok((Value::Text(value), n))
+            Ok((Value::Text(Arc::new(value)), n))
         }
     }
 }

core/types.rs

@@ -1,3 +1,5 @@
+use std::sync::Arc;
+
 use anyhow::Result;
 
 #[derive(Debug, Clone, PartialEq)]
@@ -5,7 +7,7 @@ pub enum Value {
     Null,
     Integer(i64),
     Float(f64),
-    Text(String),
+    Text(Arc<String>),
     Blob(Vec<u8>),
 }
 
@@ -27,7 +29,7 @@ impl FromValue for i64 {
 impl FromValue for String {
     fn from_value(value: &Value) -> Result<Self> {
         match value {
-            Value::Text(s) => Ok(s.clone()),
+            Value::Text(s) => Ok(s.to_string()),
             _ => anyhow::bail!("Expected text value"),
         }
     }