Add Repeat object to standard library

I've several times wanted to express a "repeat" pattern in chisel, in which a single signal is duplicated N times and concatenated together. This would be equivalent to `{N {sig}}` in Verilog or `(N-1 downto 0) => sig` in VHDL. There are several ways to do this in Chisel using Scala meta-programming, but I think it would be useful to simply add it to the standard library. That way, you could do something like

    Repeat(UInt(1, 1), 8)

And get a byte with all the bits initialized to 1.