In some programming languages, e.g. JuliaLang, function names can
contain arbitrary characters. These are represented via the string macro
`var"..."`, which allows constructing identifiers that wouldn't
otherwise parse.

These names are handled correctly by `pprof` in the FlameGraph view, but
before this commit, they would produce an invalid dot file.

This fixes the dot graph export for names that contain `"`.

…otentially harmful string labels.

Remove mistaken `escapeStringForDot()` around tag names

…orDot

This escaping requirement was previously leaking into the implementation
of `graph.go`, by writing `\n` directly instead of "\n". This meant that
if it was displayed in a different program besides `dot`, it would have
been potentially incorrectly escaped.

Now, this commit moves all the escaping directly into dotgraph, which is
more encapsulated, and also allows us to escape other characters in the
name which need escaping.

This allows tag newlines to stay center justified, and legend label
newlines to stay left justified, as they were before we started escaping
them for Dot.

Also ran gofmt