Make the regalloc module private.

Cretonne clients don't need to know how the register allocator works.

Export the RegDiversions type from the binemit module instead. It is
used by the "test binemit" driver.

cranelift/src/filetest/binemit.rs

@@ -10,7 +10,7 @@ use cretonne::binemit;
 use cretonne::dbg::DisplayList;
 use cretonne::ir;
 use cretonne::ir::entities::AnyEntity;
-use cretonne::regalloc::RegDiversions;
+use cretonne::binemit::RegDiversions;
 use cton_reader::TestCommand;
 use filetest::subtest::{SubTest, Context, Result};
 use utils::{match_directive, pretty_error};