Support spread operator ('...') on strings

[DanielRosenwasser]

• In function calls
• In array literals

For each of these, we may have to consider surrogate pairs.

[mhegazy]

Currently we do not support spread on strings. The main reason is we do not want ot do type system directed emit.

two questions:

• Should we add a helper function that extracts the array from a string in cases we do not know the type?
• Should we handel string literals?

[ahejlsberg]

@mhegazy What is the spec issue here?