wincred

• Updated dependency golang.org/x/sys to version 0.15.0
• Updated dependency github.com/stretchr/testify to version 1.8.4
• Added error constant ErrBadUsername that can be used when dealing with domain password credentials.

• Calling SyscallN directly when dealing with pointer-pointers (#42) to try to address issue #32.
• Bumped required Go version to 1.18.
• Bumped dependency to golang.org/x/sys to version 0.8.0.

• Removes the syscall package to use the golang.org/x/sys one.
• See #22 for details

• This updates the dependency to github.com/stretchr/testify

• Using error codes instead of english error strings
• Exposing two error codes as constants
• Adds go module
• Requires at least go1.13

The library can also be build on non-Windows platforms.
All operations will return a Operation not supported error on those platforms.

Corrected some usages of unsafe.Pointer according to the golang
documentation. Used go vet to verify.

This also adds and updates the package documentation.
Some examples have been added to the documentation.