This adds support for using ES6 Iterables in for...of loops. It also adds a custom decorator 'LuaIterator' to indicate a declared function is actually a proper lua iterator and should be used directly in the transpiled for...in loop.

This feature required a lot of additions and changes:

• New lib function '__TS__Iterator' which is used in place of pairs to iterate Iterables
• New lib 'Symbol' for well-known symbol support (currently only has Symbol.iterator' but more can be added in the future)
• Updated Map and Set libs to use custom iterators
• Updated lib import logic to ensure that dependencies of libs are included before the lib itself
• Forced Symbol lib to be included first in bundle, as Map and Set depend on it
• Support for methods declared with computed names
• Fixed a bug in transpileElementCall that wasn't caught by tests (and updated tests)

Something to note about LuaIterator is that it can be combined TupleReturn:

/** @luaIterator */
/** @tupleReturn */
declare function luaIter(): Iterable<[string, string]>;
for (let [a, b] of luaIter()) {}

for a, b in luaIter() do
end

But if a function is declared this way, the variable(s) in the for...of loop MUST be destructured or an error will be thrown:

for (let a of luaIter()) {} // Error - 'a' is not destructured

This is because there is no straight forward way to represent this in lua. A solution could be to wrap the iterator call in another custom lib function that bundles the results, but this is pretty tricky and I suspect most of the time this error will be hit by accident when the intention was to use the destructured version.