Enforce function declaration format in bash8

Check that function calls look like ^function foo {$ in bash8, and fix
all existing failures of that check.  Add a note to HACKING.rst

Change-Id: Ic19eecb39e0b20273d1bcd551a42fe400d54e938
diff --git a/exercises/client-args.sh b/exercises/client-args.sh
index e79774f..b360f1e 100755
--- a/exercises/client-args.sh
+++ b/exercises/client-args.sh
@@ -154,7 +154,7 @@
 # Results
 # =======
 
-function report() {
+function report {
     if [[ -n "$2" ]]; then
         echo "$1: $2"
     fi