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.
ssh: unicode adaptions
Fix mkdir race

Creating examples/code and examples/dict in parallel can fail when
examples doesn't exists. This has been seen on FreeBSD.
Merge branch 'anders/diameter/mkdir_race/OTP-11051' into maint

* anders/diameter/mkdir_race/OTP-11051:
  Fix mkdir race
Merge branch 'maint'
Merge branch 'nox/move-let-into-seq/OTP-11056'

* nox/move-let-into-seq/OTP-11056:
  Move let expressions into sequences
Update primary bootstrap
Merge branch 'fredrik/ssh/unicode_adapt' into maint

* fredrik/ssh/unicode_adapt:
  ssh: unicode adaptions
Merge branch 'maint'