Stop using Object.defineProperties in the Path constructor.
Stop using Object.defineProperties in the Path constructor.

This makes the .value, .parentPath, and .name properties no longer
immutable, which is regrettable, but it speeds up test/perf.js by a
whopping 34% (from 380ms to 250ms on my machine).

benjamn/ast-types,jamestalmage/ast-types,marcioj/ast-types,benjamn/ast-types,benjamn/ast-types,cpojer/ast-types,gabelevi/ast-types