[RFA 0/2] Support ptype/o in Rust

[Tom Tromey <tom@tromey.com>]

This adds support for ptype/o to the Rust code.

The first patch slightly refactors the existing ptype/o code.  The
utility functions are now public methods on struct print_offset_data.

The second patch changes the Rust language code.  I would self-approve
this one but it required a change outside of Rust.  Perhaps this check
ought to have been a flag on the language_defn.

I noticed that ptype/o generates somewhat funny output:

    /* offset    |  size */  type = union simple::Union {
    /*                 1 */    f1: i8,
    /*                 1 */    f2: u8,

			       /* total size (bytes):    1 */
			     }

Here, I think it might be cleaner to put the "total size" information
on the same line as the trailing "}" (and of course not indent it),
like:

    /* offset    |  size */  type = union simple::Union {
    /*                 1 */    f1: i8,
    /*                 1 */    f2: u8,
    /* total size      1 */  }

If you agree I can at least file a bug or maybe implement it.

Additionally I noticed that in C, in most cases fields are indented 4
spaces, but with ptype/o the outermost fields are only indented 2
spaces (relative to the "type =" text).  I think this is probably
unintended as well, but I thought I'd ask... ?

Regression tested on x86-64 Fedora 26.

Tom