toolset.using preserves current project

[grisumbras]

Proposed changes

using project rule preserves the current project, and toolset.using does not. The problem is that the former is not available in modules that are not projects, and so one has to preserve the current project manually, just in case. I don't think there is ever any point in not preserving the current project when initialising a toolset module, so this change pushes this reponsibility on toolset.using.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• I searched the discussions
• I searched the closed and open issues
• I read the contribution guidelines
• I checked that tests pass locally with my changes