Fix code generator bug and add tests

[stanley-cheung]

Fixes #837

The bug was caused by #780 - when fixing an issue with a missing dot, a section of the code was inadvertently removed, which handles nested messages.

So the fix is to handle both cases :)

What's challenging is to add tests to make sure both cases won't be broken by new changes to the code generator.

The test case in #837 is reflected in the test01.proto introduced by this PR.
The test case in #693 is reflected in test02.proto and test03.proto.

The tests

• run the code generator on these protos,
• run tsc on them to make sure they compile (using an entry point clientXX.ts, which imports these generated protos),
• load the compiled JS files and run some tests on them (especially on things that broken above).