escape strings to satisfy Dot syntax rules

GraphViz's "dot" language has several options for encoding IDs, as
described here: https://www.graphviz.org/doc/info/lang.html

Alga makes use of the quoted-string syntax. This permits any character
within double quotes, except the double-quote character itself, which
must be prefixed by a backslash character. (the backslash itself does
not need to be escaped).

Implement the escaping of " for defaultStyleViaShow.

Author: jmtd

src/Algebra/Graph/Export/Dot.hs

@@ -72,7 +72,9 @@ defaultStyle v = Style mempty mempty [] [] [] v (\_ -> []) (\_ _ -> [])
 -- defaultStyleViaShow = 'defaultStyle' ('fromString' . 'show')
 -- @
 defaultStyleViaShow :: (Show a, IsString s, Monoid s) => Style a s
-defaultStyleViaShow = defaultStyle (fromString . show)
+defaultStyleViaShow = defaultStyle (fromString . escape . show) where
+    escape [] = []
+    escape (x:xs) = if x == '"' then '\\':'"':(escape xs) else x:(escape xs)
 
 -- | Export a graph with a given style.
 --