Remove implicit integer conversion in SparseByteSet

Summary:
Found when tried to use `SparseByteSet`. Compiler wasn't happy,
because `-Wimplicit-int-conversion` flag was set.

```
error: implicit conversion loses integer precision: 'int' to 'uint8_t' (aka 'unsigned char') [-Werror,-Wimplicit-int-conversion]
        dense_[sparse_[i]] = last_element;
```

Reviewed By: marksantaniello, DenisYaroshevskiy

Differential Revision: D60284815

fbshipit-source-id: 5a70acd40f16bb5d31c3f361de1954a55af4d05d

folly/container/SparseByteSet.h

@@ -81,7 +81,7 @@ class SparseByteSet {
     bool r = contains(i);
     if (r) {
       if (dense_[size_ - 1] != i) {
-        int last_element = dense_[size_ - 1];
+        uint8_t last_element = dense_[size_ - 1];
         dense_[sparse_[i]] = last_element;
         sparse_[last_element] = sparse_[i];
       }