Revert "src/lstring.js: Freeze strings to catch accidental modification"
Revert "src/lstring.js: Freeze strings to catch accidental modification"

This reverts commit 5cb00bbae4f1afb5cd476a637975dd9220898e15.

It turns out that frozen objects are slower, see https://jsperf.com/object-freeze-performance

giann/fengari,daurnimator/fengari,dlbd/fengari,fengari-lua/fengari