Add Slice<T> generic struct support and fix C backend generic filtering

- Add lib/Slice.bux with Slice<T> generic struct
- Fix C backend to recognize Slice/Slice* as generic types
- Use var instead of let for uninitialized Slice<T> locals
- Selfhost loop passes, tests pass

lib/Slice.bux

@@ -0,0 +1,31 @@
+module Std::Slice {
+
+extern func bux_bounds_check(index: uint, len: uint);
+
+struct Slice<T> {
+    data: *T,
+    len: uint,
+}
+
+func Slice_FromArray<T>(arr: *Array<T>) -> Slice<T> {
+    var s: Slice<T>;
+    s.data = arr.data;
+    s.len = arr.len;
+    return s;
+}
+
+func Slice_Get<T>(self: *Slice<T>, idx: uint) -> T {
+    bux_bounds_check(idx, self.len);
+    return self.data[idx];
+}
+
+func Slice_Set<T>(self: *Slice<T>, idx: uint, value: T) {
+    bux_bounds_check(idx, self.len);
+    self.data[idx] = value;
+}
+
+func Slice_Len<T>(self: *Slice<T>) -> uint {
+    return self.len;
+}
+
+}

src/c_backend.bux

@@ -683,6 +683,7 @@ func CBE_IsGenericTypeName(name: String) -> bool {
     if String_Eq(name, "MapEntry") || String_Eq(name, "MapEntry*") { return true; }
     if String_Eq(name, "StringMap") || String_Eq(name, "StringMap*") { return true; }
     if String_Eq(name, "StringMapEntry") || String_Eq(name, "StringMapEntry*") { return true; }
+    if String_Eq(name, "Slice") || String_Eq(name, "Slice*") { return true; }
     return false;
 }