RFR 8192974: overhaul descriptor string parsing

[Maurizio Cimadamore]

Hi,
this patch is a rewriting on the layout descriptor parser code - the 
previous code was working correctly, but was a bit too convoluted and 
lacked documentation. The new code is simpler and adds the grammar 
productions in the javadoc of the various parsing methods. This work 
should make it easier to add new features to the layout descriptor DSL 
(such as symbolic references).

The new parser has a single entry point - namely 'parseLayout', which 
gives back a Stream<Type>.

I've written a big test case that generates the set of sentences that 
can be expressed in the descriptor grammar - of course since this set is 
infinite, we need to do some tricks in order to prevent infinite 
expansions of productions - and we also need to do some tricks not to 
generate too many combinations.

Currently, the test checks approx 7 millions different combinations - 
and it does so in 10 seconds and with 1G of heap, which I think it's 
acceptable.

I also had to tweak the existing DescriptorTest as that was relying on 
the structure of the old parser.

Cheers
Maurizio