Split EntityMap into entity::PrimaryMap and entity::EntityMap.

The new PrimaryMap replaces the primary EntityMap and the PrimaryEntityData
marker trait which was causing some confusion. We now have a clear
division between the two types of maps:

- PrimaryMap is used to assign entity numbers to the primary data for an
  entity.
- EntityMap is a secondary mapping adding additional info.

The split also means that the secondary EntityMap can now behave as if
all keys have a default value. This means that we can get rid of the
annoying ensure() and get_or_default() methods ther were used everywhere
instead of indexing. Just use normal indexing now; non-existent keys
will return the default value.

lib/cretonne/src/ir/mod.rs

@@ -36,20 +36,17 @@ pub use ir::types::Type;
 pub use ir::valueloc::{ValueLoc, ArgumentLoc};
 
 use binemit;
-use entity_map::EntityMap;
+use entity::{PrimaryMap, EntityMap};
 use isa;
 
 /// Map of value locations.
 pub type ValueLocations = EntityMap<Value, ValueLoc>;
 
 /// Map of jump tables.
-pub type JumpTables = EntityMap<JumpTable, JumpTableData>;
+pub type JumpTables = PrimaryMap<JumpTable, JumpTableData>;
 
 /// Map of instruction encodings.
 pub type InstEncodings = EntityMap<Inst, isa::Encoding>;
 
 /// Code offsets for EBBs.
 pub type EbbOffsets = EntityMap<Ebb, binemit::CodeOffset>;
-
-/// Map of global variables.
-pub type GlobalVars = EntityMap<GlobalVar, GlobalVarData>;