Reimplement Rust slice printing

The current nightly Rust compiler (aka 1.61) added better DWARF
representation for unsized types.  Fixing this is PR rust/21466; but
the code is actually the same as what is required to make slice
printing more useful, which is PR rust/23871.  This patch implements
this.  I tested this against various Rust compilers: 1.48, current
stable, current beta, and current nightly.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=21466
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=23871