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:

local x = {
	2 3 "string" -- no delimiters!
	z.w = 3;
	y[t[s]] = 11;
}