Simplify pinned-vreg API: don't require slice of all pinned vregs. (#28)

Simplify pinned-vreg API: don't require slice of all pinned vregs.

Previously, we kept a bool flag `is_pinned` in the `VRegData`, and we
required a `&[VReg]` of all pinned vregs to be provided by
`Function::pinned_vregs()`. This was (I think) done for convenience, but
it turns out not to really be necessary, as we can just query
`is_pinned_vreg` where needed (and in the likely implementation, e.g. in
Cranelift, this will be a `< NUM_PINNED_VREGS` check that can be
inlined). This adds convenience for the embedder (the main benefit), and
also reduces complexity, removes some state, and avoids some work
initializing the regalloc state for a run.

src/ion/data_structures.rs

@@ -259,7 +259,6 @@ pub struct VRegData {
     pub ranges: LiveRangeList,
     pub blockparam: Block,
     pub is_ref: bool,
-    pub is_pinned: bool,
 }
 
 #[derive(Clone, Debug)]