Add support for Arc<str> value

[stepantubanov]

Motivation

It's a performance optimization. Currently strings are represented by sea_query::Value::String(Box<String>) which requires a clone if the value is still needed after the query, which means the entire string will be copied. For large values it adds up to significant overhead.

Proposed Solutions

Add enum variant RcString(SeaRc<str>) to sea_query::Value.