jsonpath function always duplicates input string for unescaping

[mk6i]

The main jsonpath() function always duplicates the input string passed to the lexer so that the lexer can modify the expression without modifying the input string. The lexer currently unescapes the input string by shifting characters. This was done in order to support escaped string literals in jsonpath. Duplicating the string requires a malloc() and corresponding free(). In order to improve overall performance of the extension, let's try to avoid this string duplication.