Replace T(String) constructors with generic, variadic parse(str, ..T) function

[ramonasuncion]

Joe: Int(String) is quite a different operation from Int(Int8) or Int(Float), so it's weird presenting the same interface for both. It would be nice to have a variadic inverse operation to printTo(String, ..values) too. Both of these issues could be addressed with a parse(str, ..T) interface for reading objects from character sequences.

parse(T, str) implemented by tepancheg in (look at original issue) He proposed a better design, having the variadic parseSeparated be a different symbol.