Replace use of text/template with string manipulations

[pgimalac]

Fixes #424.

This PR replaces using a template to generate the graph in dot format with directly writing strings in the writer.
In particular it avoids disabling dead code elimination.

I tried to keep the same whitespaces (tab, newlines) as in the template to avoid modifying the test cases, and increase confidence that the change is equivalent.
I'd be very happy to remove the extra tab and newlines as they just make the code less readable.

The proposed implementation uses fmt.Fprintf to simplify, but it can be optimized by just using w.Write directly instead, avoiding some parsing and allocations. I considered that it was not a hot path so making the code simpler was worth it.

[CLAassistant]

All committers have signed the CLA.

[JacobOaks]

LGTM. For the purposes of this change, keeping the tests the same seems like a good idea to keep confidence. We can clean up the spacing later. Thanks!