Speedup __ctx_to_str.

[rchateauneu]

Fixes #

Proposed Changes

[coveralls]

Coverage remained the same at 75.705% when pulling bd96638 on rchateauneu:speedup_memory_store into 58b23fd on RDFLib:master.

[aucampia]

Looks good to me. Would be interesting to know what the speedup is here, but good change even if it provided no speedup.

[rchateauneu]

The speedup is between 0.5% and 1% when loading "orkg.nt"
g = Graph()
g.parse("orkg.nt", format="nt")

The profiles are obtained with:

py -3.6 -m cProfile -o mem.pstats import_orgk.py
py -3.6 gprof2dot.py -f pstats mem.pstats -o mem.dot
dot -Tpng -o mem.png mem.dot

FYI, using "%s:%s" is a bit faster.

[FlorianLudwig]

even without any speedup, code is cleaner, LGTM :)