[foreign-memaccess] 8227393: Add Constable support to the layout API

[Maurizio Cimadamore]

As suggested by Brian, there's no reason as to why MemoryLayout classes 
could not implement the Constable interface. After all, they are static 
description of some memory layout, and a constant pool form for them can 
be defined (using condy). This patch does just that.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8227393/

Now, there are some degree of freedom in doing this mapping; from other 
cases, I infer that the idiomatic way to implement constable would be to 
return a custom DynamicConstantDesc (e.g. enums return an EnumDesc, 
VarHandles a VarHandleDesc) - but in this case I'm not sure the exercise 
is worth it for two reasons:

* I can't see any 'interesting method' that could be added to a 
LayoutDesc - seems simpler to just work on the layouts themselves

* To canonicalize dynamic constants representing layouts, we'd still 
need some behavioral changes in the public SE API - which is a bit 
tricky given everything is defined in an incubator module

I'd say that what I have implemented is good enough for now, but of 
course we should reserve the right to change it later if/when the code 
ends up in java.base.

Similar considerations hold for memory access VarHandles Constable 
support - on the one hand we could just add such support - on the other 
hand, to do that correctly we'd need some surgery in VarHandleDesc - 
which is again a SE API in java.base. I think when the time comes, we'd 
add a new 'kind' to VarHandleDesc called MEMORY, and then add a suitable 
implementation of describeConstable under VarHandleMemoryAddressBase (as 
well as canonicalize support).

Note: I had to add a 'missing' BSM constant in ConstantDescs - namely 
for ConstantBootstraps#getStaticFinal. I hope this change is 
innocuous/standalone enough.

Cheers
Maurizio