feat!: allow for references contract traits

[willemneal]

Allows external interface to use values and then the internal contracttraits can use references.

#[contracttrait]
pub trait Foo {
     fn foo(e: &Env, s: &String);
}

// Generated exported methods will use value instead of reference

impl Contract {
  pub foo(e: &Env, s: String) {
      <Contarct as ContractTrait>::foo(e, &s)
  }