Derive macros for stargate-grpc

This crate provides the following derive macros:

• IntoValue – enables converting a Rust struct to a Value of a user-defined CQL type; use this when you want to bind a single UDT field in a query
• TryFromValue – enables converting a Value representing a user-defined CQL type to a Rust struct; use this to read a single UDT column value from a row
• IntoValues – enables converting a Rust struct to many arguments of a query at once;
  use this if you want to bind many fields of a single Rust struct in a single call to bind
• TryFromRow – enables converting a Row received in a result set to a Rust value

Example

```rust,skt-simple-main use stargategrpc::Value; use stargategrpc_derive::{IntoValue, TryFromValue};

[derive(IntoValue, TryFromValue)]

struct User { id: i64, login: String }

let user = User { id: 1, login: "user".to_string() };

// Convert User to Value: let value = Value::from(user); assert_eq!(value, Value::udt(vec![("id", Value::bigint(1)), ("login", Value::string("user"))]));

// Now convert it back to User: let user: User = value.tryinto().unwrap(); asserteq!(user.id, 1); asserteq!(user.login, "user".tostring());

```

See crate documentation for more examples.