Combinators for Show

[Li-yao Xia]

Hello libraries,

base contains a small combinator library for Read, but there doesn't 
seem to be a similar thing for Show. Offering a few more convenient 
helpers to avoid dealing with whitespace and especially precedence 
levels seems straightforward enough.

Is there an API somewhere that I missed? Has there been a 
proposal/discussion about this before? I'm ready to make one otherwise.

Li-yao