Fix several C++ warnings (#1565)

Summary:
Fixes warnings about implicit type truncation. By making the conversions explicit, react-native-windows will be able to remove suppressions for several warnings required per SDL.

Pull Request resolved: https://github.com/facebook/folly/pull/1565

Reviewed By: igorsugak

Differential Revision: D27964258

Pulled By: yfeldblum

fbshipit-source-id: b13f8728ecda0b38c2fd661199d6551c5ff87993

folly/dynamic.cpp

@@ -297,7 +297,7 @@ std::size_t dynamic::hash() const {
           [&](auto acc, auto const& item) { return acc + h(item); });
     }
     case ARRAY:
-      return folly::hash::hash_range(begin(), end());
+      return static_cast<std::size_t>(folly::hash::hash_range(begin(), end()));
     case INT64:
       return std::hash<int64_t>()(getInt());
     case DOUBLE:

folly/lang/ToAscii.h

@@ -75,7 +75,7 @@ struct to_ascii_array {
     data_type_ result{};
     Alphabet alpha;
     for (size_t i = 0; i < Base; ++i) {
-      result.data[i] = alpha(i);
+      result.data[i] = alpha(static_cast<uint8_t>(i));
     }
     return result;
   }
@@ -224,7 +224,7 @@ FOLLY_ALWAYS_INLINE size_t to_ascii_size_clzll(uint64_t v) {
   }
 
   //  log2 is approx log<2>(v)
-  size_t const vlog2 = 64 - to_ascii_port_clzll(v);
+  size_t const vlog2 = 64 - static_cast<size_t>(to_ascii_port_clzll(v));
 
   //  handle directly when Base is power-of-two
   if (!(Base & (Base - 1))) {