haskell-translator: avoid invalid escape sequence warnings

"\<xsymbol>" is not a valid string in python, although the compiler
generally accepts it and does the right thing. More recent versions warn
about it. To avoid that noise, turn these into either r"\<xsymbol>", or
"\\<xsymbol>" when the former is not possible.

Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>