Add pure/const/@nogc/nothrow attributes

[araspik]

Stuff like sdlang.ast.Tag.parent, which are @property, can generally also have pure, nothrow, @nogc, and const. Please consider adding these attributes globally so that others can use them too!

[ZILtoid1991]

I'm already on this. Currently purity is hard to achieve thanks to dependencies not being updated for years, and often due to similar reasons, I've to resort to @trusted instead of @safe. Currently I'm dealing with some issues from unittest, but that might be something that was always there (how is the lexing of floating point numbers?).