fix misdetection of global variables (issue #4 cont'd)

in table accesses like `x:y().z` (`prefixexp` in Lua grammar) y and z are not references to variables, global or local; they are table keys. likewise in table constructors like `local x = { y = { z = 5 } }`

also, fix mistakenly allowing code like:

```lua
local x = {
	2 3 "string" -- no delimiters!
	z.w = 3;
	y[t[s]] = 11;
}
```