opaque `struct value'

[Andrew Cagney]

Hello,

I'm going to clean up "struct value" so that it's opaque (just like I've 
previously implemented with struct frame et.al.).  I intend simply 
committing any mechanical transformations (s/macro/function), while more 
complex ones (e.g., changing an interface), will be separated out. 
Doing this will give us the control we need to implementing things like 
DW_OP_piece.

Comments?

First off the bat, I'd like to fix the file name.   values.c should be 
called value.c.

Comments?

Andrew