Update version numbers for R17A development
Update primary bootstrap
Update preloaded modules
Move let expressions into sequences

The Core Erlang compiler simplifies let expressions by moving them
into their argument when it is another let or a case expression.
It can then perform other optimizations such as removing the let
expressions altogether, sometimes saving BEAM registers later in
the process.

This commit teaches sys_core_fold how to move let expressions into
sequence arguments.
Update kernel.appup.src for R17
Update stdlib.appup.src for R17
Update sasl version and sasl.appup.src for R17

Also clean up appup_test in sasl_SUITE by removing old versions that
are no longer used.
Update "old vsn" in sasl test to R15 instead of R14 (two back from R17)
Update primary bootstrap
Fix reltool.copy_file() a possible error - "not owner".