Ensure &await can handle flattenable Iterables

Fixes #1323

We don't want to do deconts for the user, as previous commits
tried to fix the issue with, but we still want[^1] to accept whatever
would be accepted by *@ slurpy:

- 6.c: eagerize the map, so nested levels get reified
- 6.d: toss Iterable and Any:D candidates to force these things
    go through the slurpy

Added the test for throwage on `await non-Iterable` to make test
instead of roast, cause of [2].

[1] https://irclog.perlgeek.de/perl6-dev/2017-12-20#i_15604469
[2] https://irclog.perlgeek.de/perl6-dev/2017-12-20#i_15604448

src/core.d/await.pm

@@ -15,9 +15,6 @@ multi sub await() {
 multi sub await(Any:U $x) {
     die "Must specify a defined Awaitable to await (got an undefined $x.^name())";
 }
-multi sub await(Any:D $x) {
-    die "Must specify an Awaitable to await (got a $x.^name())";
-}
 multi sub await(Awaitable:D \a) {
     CATCH {
         unless nqp::istype($_, X::Await::Died) {
@@ -26,14 +23,6 @@ multi sub await(Awaitable:D \a) {
     }
     $*AWAITER.await(a)
 }
-multi sub await(Iterable:D \i) {
-    CATCH {
-        unless nqp::istype($_, X::Await::Died) {
-            ($_ but X::Await::Died(Backtrace.new(5))).rethrow
-        }
-    }
-    $*AWAITER.await-all(i)
-}
 multi sub await(*@awaitables) {
     CATCH {
         unless nqp::istype($_, X::Await::Died) {

src/core/asyncops.pm

@@ -50,8 +50,8 @@ multi sub await(Supply:D $s) {
     my $*RAKUDO-AWAIT-BLOCKING := True;
     $*AWAITER.await($s)
 }
-multi sub await(Iterable:D $i) { $i.eager.map({ await $_ }) }
-multi sub await(*@awaitables)  { @awaitables.eager.map({await $_}) }
+multi sub await(Iterable:D $i) { eager $i.eager.map({ await $_ }) }
+multi sub await(*@awaitables)  { eager @awaitables.eager.map({await $_}) }
 
 sub awaiterator(@promises) {
     Seq.new(class :: does Iterator {