We were using sw, the namespace used in the rest of SwiftShader, as the
namespace for Reactor. Putting Reactor code into its own namespace
makes it easier to untangle dependencies.

Bug swiftshader:16
Bug b/115344057

Change-Id: Iea4e049a4796323bf80b4f5e6e17778ccf17b995
Reviewed-on: https://swiftshader-review.googlesource.com/c/21308Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>