When tstl transpiles a function expression or arrow function, it attempts to infer whether it should be transpiled with a "self" parameter or not, based on what it is being assigned to.

Currently, this does not work in complex scenarios where function expressions were in array or object literals:

interface Foo {}
interface FooMethod {
    (this: Foo, s: string): string;
}
const methodArray: FooMethod[] = [s => s]; // Used to be an error

interface Foo {
    bar(this: void, s: string): string;
}
const foo: Foo = {
    bar: s => s // Used to be an error
};

This PR adds support for properly inferring the self type in these situations and prevents those annoying function/method conversion errors.

also fixes #340