Add indent parameter to Serializer.serialize() (#2177)

Using Utils.indent() gives deprecation warnings to use Serializer instead. However,
the Serializer class itself doesn't provide a means to manually indent a FirrtlNode
string a certain number of times.

The indent variable, previously hardcoded to 0, is now exposed as a second parameter
for the modified serialize function, and the old serialize function just calls the
modified serialize with indents = 0 for binary compatibility

Co-authored-by: Megan Wachs <megan@sifive.com>