Fail early when a top level statement fails to resolve

[DuncanUszkay1]

Issue: #1165

There's more context in the issue itself. Basically we could fail to compile a global top level statement, and we would just keep going compiling the next statement. This caused an assertion failure.

This PR makes the compileTopLevelStatement return a boolean to indicate that it has failed to compile, which is then used to halt compilation early and show an error message. The advantage of this approach is that we don't run into assertion failures later on, and instead show an error that is descriptive to the user.

Example

const INNER: fake = 1;
const OUTER: i32 = INNER;

On master we get:

ERROR: AssertionError: assertion failed
    ...stacktrace

With this branch we get:

ERROR TS2304: Cannot find name 'fake'.

 const INNER: fake = 1;
              ~~~~
 in main.ts(5,14)

ERROR: 1 compile error(s)
    at Object.main (/Users/duncanuszkay/src/github.com/Shopify/as-fork/assemblyscript/cli/asc.js:614:21)
    at /Users/duncanuszkay/src/github.com/Shopify/as-fork/assemblyscript/bin/asc:21:47

[dcodeIO]

Suggesting to fix the assertion at the place it originates, since otherwise we can't provide proper error highlighting (of all errors to fix in a file) in IDEs. The red markers would just stop at some point.

[DuncanUszkay1]

Ah Right, I wasn't thinking about the IDE case. I'll try a different approach then