add version of trueDualPortBlockRam that sets the initial RAM contents

[cuddlefishie]

This PR renames the existing trueDualPortBlockRam function to trueDualPortBlockRamU to indicate that the initial BRAM contents are undefined.

This PR also adds a new function, trueDualPortBlockRam to the prelude which takes the initial RAM contents as an argument. This function is implemented as a blackbox for HDL targets.

Fixes #2199

Still TODO:

• Write a changelog entry (see changelog/README.md)
• Check copyright notices are up to date in edited files
• Write unit-tests for trueDualPortBlockRam

[DigitalBrains1]

Looks good! I quickly went through. I'd like to suggest slightly different documentation as indicated below.

[lmbollen]

The currently existing unit tests don't check the correctness of the implementation, but rather that the generated HDL behaves the same as the Haskell model.

I think we should change this at some point.