[thin_metadata_pack/unpack] Replace C++ implementation with a Rust one.

The Rust implementation is multithreaded, performs better in general and
does custom compression of btree nodes to achieve much better compression
ratios.  unpack also checksums expanded metadata to validate it.

Format version has jumped to 3, no backwards compatibility, but I think
that's ok since we never made a release that contained the C++ version
of these tools.

Benchmarks
==========

On an 8 core, 16 hyperthread machine.

metadata 1G, full:

      Pack size    pack time     unpack time
------------------------------------------------------
C++      193M        50.3s          6.9s (no verify)
Rust      70M         1.4s          1.8s (verify)

metadata 16G, sparse:

       Pack size    pack time     unpack time
------------------------------------------------------
C++      21M          68s           1s   (no verify)
Rust      4M           8.6s         0.5s (verify)

README.md

@@ -13,6 +13,12 @@ The [expat](http://expat.sourceforge.net/) xml parser library (version 1).
 The libaio library (note this is not the same as the aio library that you get by linking -lrt)
 make, autoconf etc.
 
+A couple of non-essential tools are written in rust, and will
+require cargo and rustcc to be installed:
+
+    thin_metadata_pack
+    thin_metadata_unpack
+
 There are more requirements for testing, detailed below.
 
 Building
@@ -23,6 +29,11 @@ Building
     make
     sudo make install
 
+Building Rust tools
+===================
+
+    sudo make install-rust-tools
+
 Quick examples
 ==============