[sgen-canaries] Move to single (after-object) canary.

[sgen-canaries] verify_nursery() fixed
[sgen-canaries] Small fixes by @schani
just a bit of cleanup (and @kumpera's feedback changes) #wip
[sgen] Fix build with canaries disabled.
[sgen] Fix a bug introduced by canaries.
[sgen] Fix canary macros.
[sgen] Arithmetic on void pointers is not allowed.
[sgen] Fix String.InternalSetLength.
[sgen] Fix a canary fail printf.
[sgen] Fix canary pinning bug.