Add exported map keys to local scope

[irh]

This PR fixes an inconsistency in behaviour between exporting values using assignment or maps.

x = -1
export x = 123 
print x # The local value of `x` gets updated by the assignment export
# 123

y = -1
export {y: 99}
print y # The local of value of `y` wasn't updated by the map export
# -1

This happens because map entries typically don't get added to the local scope. The runtime will look for a local value first, and then check the exports map if one isn't found. In this case, a local y is available, so the exported value isn't used. Exporting via a map is available for convenience and shouldn't have a difference in behaviour, so this PR ensures that map-export entries get assigned locally, updating existing variables if they exist.