binding: Do not allocate vector when retrieving matching indexes from bind markers metadata

[muzarski]

Currently, CassStatement::bind_multiple_values_by_name accepts a slice of usizes. The caller creates a vector from an iterator, and passes the reference to this vector. The allocation could be omitted there, by passing an iterator directly to bind_multiple_values_by_name. The issue is, however, that the aforementioned iterator borrows from &self, and bind_multiple_values_by_name accepts &mut self.