Current string interpolation codegen can cause exponential runoff in certain Lua parsers

[lizreu]

This isn't an issue with any Lua interpreters that I know of, but libraries implementing Lua parsers (especially PEG) really struggle parsing expressions of the following form:
return (((((((((((((((("a" .. a) .. ",") .. b) .. ",") .. c) .. ",") .. d) .. ",") .. e) .. ",") .. f) .. ",") .. g) .. ",") .. h) .. ",") .. i

This is the output of something like a{a},{b},{c},{d},..

While this is trivial to fix in controlled code by not using string interpolation for very long expression, it is not trivial to fix in certain parser architectures, and impossible to guard against "in the wild".

Here is an example of a Lua parser that gets tripped up by this: Kampfkarren/full-moon#279

This -should- be straightforward to fix by flattening out the nested expression, and simply emitting "a" .. a .. "," .. b .. "," ..