Implement value_print for Rust

This adds an implementation of the value_print method to Rust.  As
described in PR rust/22254, this removes a bit of weird-looking output
from some "print"s -- because c_value_print is bypassed.  I don't have
a test for the bug that inspired this patch, because I only know how
to reproduce it when using a relatively old Rust compiler.  However,
the new "cast-printing" code in value_print is required, because
omitting this causes some existing tests to fail.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=22254

gdb/rust-lang.h

@@ -125,6 +125,11 @@ public:
 
   /* See language.h.  */
 
+  void value_print (struct value *val, struct ui_file *stream,
+		    const struct value_print_options *options) const override;
+
+  /* See language.h.  */
+
   struct block_symbol lookup_symbol_nonlocal
 	(const char *name, const struct block *block,
 	 const domain_enum domain) const override