Destructuring assignment

[degory]

Add support for destructuring assignment from tuples both for assignment statements and for variable initializers of let statements.

Even non-recursive destructuring of a single tuple into a flat list of variables would be very useful, but recursive destructuring is probably fairly easy to implement.

Destructuring of lists is probably less useful and is not a priority.

We should support placeholders for values we want to discard, e.g. _

Destructuring from:

• ValueTuple<T0, T1, T2, ...>
• By naming convention `0, `1 , `2, ...
• By deconstruct(e0: T0 ref, e1: T1 ref, e2: T2 ref, ...)

Destructuring into:

• flat parentheses delimited lists of variables in assignments
• flat parentheses delimited lists of variables in let statement variable initializers
• flat parentheses delimited lists of variables in for statement variable initializers
• nested parentheses delimited lists of variables in assignments
• nested parentheses delimited lists of variables in let statement variable initializers
• nested parentheses delimited lists of variables in for statement variable initializers

Placeholders:

• Ignore source values that map to the placeholder variable name _ on the left hand side of assignments and initializers