Golang: fix flow from a map value via a range statement

[smowton]

Flow such as the following was broken:

var someMap map[string]string
someMap["someKey"] = source()

for _, val := range someMap {
  sink(val)
}

This is because the store-step relating to the map store used content-kind "map.value", while the read via the range statement used content-kind "array".

Note the case for reading array content now always constrains the read node to have array or slice type, as array content was previously being read from the base over a range statement iterating over a map.

[smowton]

DCA: no changes.

[owen-mc]

Good catch! How did you notice this? (Edit: I just found where it was reported.) Could you add a test that failed before and now passes?

[mbg]

@owen-mc FYI, @smowton asked me to review this yesterday and I ended up writing a test for the review which seems to suggest this isn't working (unless I have done something silly). I should have commented that on here rather than on Slack.