Silence warnings for expressions that are assigned to "_"

There is currently no zero-cost way to silence the warning
"the result of the expression is ignored", which is issued
for code such as:

  list_to_integer(S),
  ok

Such code can be useful for assertions or input validation.

Teach the compiler to silence the warning for expressions
that are explicitly assigned to to the "_" variable,
such as:

  _ = list_to_integer(S),
  ok

Implement it by having the v3_core pass annotate calls in
Core Erlang like this:

  let <_> = ( call 'erlang':'list_to_integer'(S) -| ['result_not_wanted'] )
      in 'ok'

and modifiy sys_core_fold to suppress the warning for any call
having the annotation.

We deliberately do not make it possible to silence the warnings
for expressions like:

  {build,an,unnecessary,term}, ok

or

  is_list(L), ok

because we don't know of any real-world scenarios in which that would
be useful.

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