Make Generated Builders Vec-Aware

[rcoh]

Builds towards #85

Description of changes:
This updates the generated builders & fluent client to be vec & hashmap aware. The builder method with the same name as the field will append the field to the list, creating it if it doesn't exist. A set_xyz method is added which retains the original behavior. A similar behavior is generated for methods containing HashMaps

Before:

    let mut expr_attrib_names = HashMap::new();
    expr_attrib_names.insert("#yr".to_string(), "year".to_string());
    let mut expr_attrib_values = HashMap::new();
    expr_attrib_values.insert(":yyyy".to_string(), AttributeValue::N(year.to_string()));
    client
        .query()
        .table_name(table_name)
        .key_condition_expression("#yr = :yyyy")
        .expression_attribute_names(expr_attrib_names)
        .expression_attribute_values(expr_attrib_values)

After:

    client
        .query()
        .table_name(table_name)
        .key_condition_expression("#yr = :yyyy")
        .expression_attribute_names("#yr", "year")
        .expression_attribute_values(":yyyy", AttributeValue::N(year.to_string()))

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[aajtodd]

LGTM!