Add support for the sizeof function in Rust

2016-10-29  Manish Goregaokar  <manish@mozilla.com>

gdb/ChangeLog:
    * rust-exp.y: Parse `sizeof(exp)` as `UNOP_SIZEOF`

gdb/testsuite/ChangeLog:
    * gdb.rust/simple.exp: Add tests for `sizeof(expr)`