Add instance evaluation and methods to read alloc

The instance evaluation is needed to handle intrinsics such as
`type_id` and `type_name`.

Since we now use Allocation to represent all evaluated constants,
provide a few methods to help process the data inside an allocation.

compiler/stable_mir/src/target.rs

@@ -0,0 +1,50 @@
+//! Provide information about the machine that this is being compiled into.
+
+use crate::compiler_interface::with;
+
+/// The properties of the target machine being compiled into.
+#[derive(Clone, PartialEq, Eq)]
+pub struct MachineInfo {
+    pub endian: Endian,
+    pub pointer_width: MachineSize,
+}
+
+impl MachineInfo {
+    pub fn target() -> MachineInfo {
+        with(|cx| cx.target_info().clone())
+    }
+
+    pub fn target_endianess() -> Endian {
+        with(|cx| cx.target_info().endian)
+    }
+
+    pub fn target_pointer_width() -> MachineSize {
+        with(|cx| cx.target_info().pointer_width)
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq)]
+pub enum Endian {
+    Little,
+    Big,
+}
+
+/// Represent the size of a component.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
+pub struct MachineSize {
+    num_bits: usize,
+}
+
+impl MachineSize {
+    pub fn bytes(self) -> usize {
+        self.num_bits / 8
+    }
+
+    pub fn bits(self) -> usize {
+        self.num_bits
+    }
+
+    pub fn from_bits(num_bits: usize) -> MachineSize {
+        MachineSize { num_bits }
+    }
+}