Make stock strings rusty

This converts the stock strings API to be more safe-rust style.  The
API is kept roughly the same for now but moved to methods on the
context.

src/lib.rs

@@ -34,6 +34,7 @@ pub mod peerstate;
 pub mod pgp;
 pub mod smtp;
 pub mod sql;
+pub mod stock;
 pub mod types;
 pub mod x;
 
@@ -60,9 +61,11 @@ pub mod dc_receive_imf;
 pub mod dc_saxparser;
 pub mod dc_securejoin;
 pub mod dc_simplify;
-pub mod dc_stock;
 pub mod dc_strencode;
 pub mod dc_token;
 pub mod dc_tools;
 
 pub use self::constants::*;
+
+#[cfg(test)]
+pub mod test_utils;