Support += with record

[phated]

Given the following code, I receive this parser error:

Syntax error after 'idx' and before '+='.
Expected an infix operator to continue the expression or a newline character to terminate it.

record Foo {
    mut idx: Number
}

let foo = { idx: 1}

foo.idx += 1

I believe the += operator should allowed to be used on mutable record fields.