* refactor: replace `forEach` with `for`-loops
Changes:
* Most `Object.keys(o).forEach` are replaced by `for in`.
* Most `Array.filter(c).forEach` are replaced by `for of` + `if continue`.
* `return` in `forEach` callbacks are replaced by `continue`.
There may be minor improvement to memory footprint as this change would
reduce the creations of temporary arrays and temporary functions.
* fix: return -> continue
* refactor: remove the commented out code
