ByteCodeGen: use depth instead of offsets in BCEnv

This is based on unfinished work in D38 started by Simon Marlow and is
the first step for fixing #13825. (next step use byte-indexing for
stack)

The change boils down to adjusting everything in BCEnv by +1, which
simplifies the code a bit.

I've also looked into a weird stack adjustement that we did in
`getIdValFromApStack` and moved it to `ByteCodeGen` to just keep
everything in one place. I've left a comment about this.

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

Test Plan: ./validate

Reviewers: austin, hvr, bgamari, simonmar

Reviewed By: bgamari, simonmar

Subscribers: simonmar, rwbarton, thomie

GHC Trac Issues: #13825

Differential Revision: https://phabricator.haskell.org/D3708