escape backslash in key values in elemToString

[jmestwa-coder]

elemToString escapes = and ] in a PathElem key value but not the backslash that extractKV uses as its escape character, so the two disagree. A value like a\b serialized by PathToString re-parses to ab, and a trailing backslash lets one key absorb the next, so {k1:"a", k2:"b"} round-trips to a single k1="a][k2=b]". Escape the backslash first so PathToString output round-trips through StringToStructuredPath.

[coveralls]

coverage: 88.728%. remained the same — jmestwa-coder:elemtostring-escape-backslash into openconfig:master