The BSD-style license that Go uses requires the license to be included
with the source distribution; so add it as LICENSE.wasmexec (to avoid
confusion with the other licenses in rclone) and note the location of
the license in wasm_exec.js itself.
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.
