Avoiding allocating a handle in the Windows RNG.

[teo-tsirpanis]

Issue #, if available:

Description of changes:

This PR uses the BCRYPT_USE_SYSTEM_PREFERRED_RNG option in the call to BCryptGenRandom to avoid allocating an algorithm handle, and simplifying the code a bit.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jmklix]

Can you please explain why you would want to avoid allocating an algorithm handle?

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[teo-tsirpanis]

Can you please explain why you would want to avoid allocating an algorithm handle?

It reduces global state and slightly simplifies the code. This approach is being used by OpenSSL and .NET among others.

[graebm]

Thanks!