Make printer more flexible, allow very-compact forms

[rvagg]

I'm attempting here to make something that lets me see the forms of a block without looking at the matrix and without it getting too out of hand when the block is complex.

See printer/printer_test.go for examples of what this is doing. There's some breakage here, however, so @warpfork you might want to weigh in if you still care about the printer package:

• Always add commas between list/map items (except for the last one)
• Remove the space between the index/key and the value (so it's always like foo:string{"bar"} or 10:string{"bar"})
• Don't change Indentation if it's left nil, it just means there's no indentation, you have to set it. The default package-exposed functions set it to \t though.
• If Indentation is nil, then we also don't print newlines; similar to the behaviour of encoding/json and also the JS JSON.stringify.
• Add an OmitScalarValues option

As per test example, a basic Config{OmitScalarValues: true} would give you the most compact form:

map{string:string,string:string,string:map{string:string,string:string},string:list{0:int,1:int},string:list{0:float}}

So far, the most complex thing I've printed with this is a Filecoin HAMT root with embedded SectorPreCommitOnChainInfo (tuple representation) values in it:

list{0:bytes,1:list{0:list{0:list{0:bytes,1:list{0:list{0:int,1:int,2:link,3:int,4:list{},5:int,6:link},1:bytes,2:int}},1:list{0:bytes,1:list{0:list{0:int,1:int,2:link,3:int,4:list{},5:int,6:link},1:bytes,2:int}}},1:list{0:list{0:bytes,1:list{0:list{0:int,1:int,2:link,3:int,4:list{},5:int,6:link},1:bytes,2:int}},1:list{0:bytes,1:list{0:list{0:int,1:int,2:link,3:int,4:list{},5:int,6:link},1:bytes,2:int}}},2:list{0:list{0:bytes,1:list{0:list{0:int,1:int,2:link,3:int,4:list{},5:int,6:link},1:bytes,2:int}}},3:list{0:list{0:bytes,1:list{0:list{0:int,1:int,2:link,3:int,4:list{},5:int,6:link},1:bytes,2:int}}},4:list{0:list{0:bytes,1:list{0:list{0:int,1:int,2:link,3:int,4:list{},5:int,6:link},1:bytes,2:int}}}}}

It's still a little bit like looking in the matrix but it's more helpful than printing out CBOR diagnostic output and I'm yet to properly build a set of schemas to match these against.

[rvagg]

One adjustment that might be worth making here is adding back the space after : when there is indenting, otherwise compress it. I'm fine with this as it is but encoding/json and JSON.stringify also have this behaviour I think.