Specify the location of arguments as well as the size of stack argument
array needed. The ABI annotations are optional, just like the value
locations.
Remove the Eq implementation for Signature which was only used by a
single parser test.
These two tables are used to keep track of type signatures of function
calls as well as external function references used in direct function
calls.
Also add an ExtFuncData struct representing an external function that
can be called directly.
Give these crates each a more standard directory layout with sources in
a 'src' sub-sirectory and Cargo.toml in the top lib/foo directory.
Add license and description fields to each.
The build script for the cretonne crate now lives in
'lib/cretonne/build.rs' separating it from the normal library sources
under 'lib/cretonne/src'.
