change PAST comment references to QAST in all source code

[tbrowder]

Also change variables named ‘$past’ to ‘$qast’.

[zoffixznet]

Let's not make frivolous changes all over the source. It makes git blame more annoying to use and makes merges of branches harder.

That already caused a lot of trouble just two days ago:

16:05 | nine | Oh the joy of fixing merge conflicts because of whitespace cleanup :/ 
https://github.com/perl6/nqp/commit/86362f5c3c211acc09bc0dbdb592d7d3f52637bf
16:06 | nine | For future reference: please just don't


16:41 | jnthn | Yeah, I was annoyed by that PR doing so called "code quality improvements", 
which mostly meant `final` clutter, whitespace changes, style changes that aren't better just 
different...I didn't see a single useful thing in there. Wish I'd not been distracted with other 
things and could have shot the PR down before it got merged :/
16:42 | jnthn | Maybe we need a tighter commit policy on NQP, as MoarVM and Rakudo have.

[jnthn]

Yeah, I'd say the comments are maybe OK as one doesn't care much for those in blame; for the $past -> $qast thing, let's just do it when we're touching bits of code using them anyway for other reasons.

[tbrowder]

Okay.