[4.0] Add explicit operators for Color4/Vector4 conversions

[devvoid]

Purpose of this PR

• Adds operators to convert between Color4 and Vector4.
• Affects OpenTK.Mathematics
• No extra documentation needed.

Testing status

• Code has been benchmarked to ensure this is the fastest way to do it. Results are in Color4 and Vector4 should be compatible #833
• Builds with no errors, works in both Debug and Release.

Comments

• Operation is only unsafe in a technical context, which is why it's wrapped in a big unsafe block.
• LayoutType is added to Color4 just for consistancy's sake with other types.
• Closes Color4 and Vector4 should be compatible #833.

[varon]

Solid work. Looks good. Just needs to drop the fixed keyword and clarify the docs a bit.

[varon]

Thanks for all of the super prompt changes. Nice work on these.

[devvoid]

No problem