Inline hash_table::calculate_offsets, used by iterators.

The `HashMap` and `HashSet` iterators use `RawTable::first_bucket_raw` which is generic and will get inlined cross-crate.
However, `first_bucket_raw` calls `calculate_offsets` and the call doesn't get inlined, despite being a simple function.
This missing `#[inline]` results in `hash_table::calculate_offsets` showing up at the top of a callgrind profile with 3 million calls (for the testcase in #25916).

src/libstd/collections/hash/table.rs

@@ -528,6 +528,7 @@ fn test_rounding() {
 
 // Returns a tuple of (key_offset, val_offset),
 // from the start of a mallocated array.
+#[inline]
 fn calculate_offsets(hashes_size: usize,
                      keys_size: usize, keys_align: usize,
                      vals_align: usize)