Block diagram: fixes and improved looks

- Move background to its own layer
- Make font sizes consistent
- Fix icache and pc background
  Previously the background was morphed around the text, this makes it a
  background again.
- Remove redundant rectangle
  The instruction memory interface had two rectangles, one black and one
  purple. I removed the purple one that was bleeding through in the
  corners.
- Instruction fetch alignment
  The Instruction fetch block was not the same height and was not top
  aligned with the other blocks.
- Align text with boxes
  This essentially aligns all the text insides the blocks
- Standardize lines as 0.265mm
  The lines between blocks and the ones making the triangular shapes were
  mostly 0.265mm with a few exceptions.
- Stroke width of block outlines same
  Made all the stroke widths for all the blocks 0.5mm. I've made the outer
  box a nice round 1.0mm.
- Use lowRISC colors
  E0384F for the background (including the start of the gradient)
  A21F4F for the outside line
- Alignment of in/out arrows
  Many of these arrows were not aligned, this improves that alignment.
- Add white background to instr inf
  Instruction memory interface lost its white background when the purple
  outline was removed. This commits adds it back in.
- Use Liberation Sans everywhere
  Exo 2 is not supported natively in browsers and there was no easy way to
  embed fonts in SVG where Inkscape knew about it.
- Fade to white, not transparent
- PMP check font is now smaller
- Add background to debug request input
- Make text under prefetcher bigger so it is rendered on GitHub
- Execute text is now its own block so that it is rendered on GitHub