Solves link error while running rclone's wasm version. Go's `walltime1` function was renamed to `walltime`. This commit updates wasm_exec.js with the new name.
This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.
This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.
