Fix Issue 10297 - Memory safe D spec is out of date

Link to spec from front page instead of safed.html article.
Replace 'Proscribed Forms' section with link to @safe function
attribute.
Introduce @safe, @trusted.
Add note about array bounds checks.
Remove unimplemented -safe switch
List a few examples of unsafe operations
Merge pull request #600 from ntrel/update-safe-spec

Fix Issue 10297 - Memory safe D spec is out of date