Merge 'Add jsonb bench and rewrite jsonb parser to use slice and cursor' from Ihor Andrianov

Added bench to see how far i am from SQLite. If ~~Cloude~~ I did benches
correct moving from iterator to slice and cursor gives crazy performance
bump where we are almost on par with SLQLite when we are not talking
about huge jsons, but i suspect SQLite do some trickery there.

Closes #1125

core/Cargo.toml

@@ -108,3 +108,7 @@ harness = false
 [[bench]]
 name = "mvcc_benchmark"
 harness = false
+
+[[bench]]
+name = "json_benchmark"
+harness = false

core/json/mod.rs

@@ -75,7 +75,7 @@ pub fn get_json(json_value: &OwnedValue, indent: Option<&str>) -> crate::Result<
 pub fn jsonb(json_value: &OwnedValue) -> crate::Result<OwnedValue> {
     let jsonbin = match json_value {
         OwnedValue::Null | OwnedValue::Integer(_) | OwnedValue::Float(_) | OwnedValue::Text(_) => {
-            Jsonb::from_str(&json_value.to_string())
+            Jsonb::from_str(&json_value.to_text().unwrap())
         }
         OwnedValue::Blob(blob) => {
             let blob = Jsonb::new(blob.len(), Some(&blob));