Casting integral structs, and coercions

[Jose E. Marchesi]

Hi people!

In the last pokology I mentioned I was working on adding support for
casting integral structs to integers.  That support is ready now in
master.

Let's use the following type as an example:

deftype Elf64_RelInfo =
  struct Elf64_Xword
  {
    uint<32> r_sym;
    uint<32> r_type;
  };

There are two ways to handle integral structs as integers:

1. Using explicit casts.  For example:

   (poke) defvar info = Elf64_RelInfo { r_sym = 3 }
   (poke) info
   Elf64_RelInfo {
     r_sym=0x3U,
     r_type=0x0U
   }
   (poke) info as uint<64>
   0x300000000UL

2. Integral structs are automatically coerced to an integer in contexts
   where integer values are expected.  This includes arithmetic
   opeators, passing arguments to functions, returning values from
   functions, conditions in if, while, for-in-where statements, etc.
   Examples:

   (poke) +info
   0x300000000UL
   (poke) -info
   0xfffffffd00000000UL
   (poke) defun getinfo = uint<64>: { return Elf64_RelInfo {}; }
   (poke) getinfo
   0x0UL
   (poke) Elf64_RelInfo { r_sym = 2 } + Elf64_RelInfo { r_sym = 3 }
   0x500000000UL

Next step is to add support for casting integers to integral structs,
i.e.:

(poke) 0x300000000 as Elf64_RelInfo
Elf64_RelInfo {
  r_sym=0x3U,
  r_type=0x0U
}

Salud!