Parallel Hashmap Versions Save

Small changes, such as windows compilation flags, and renaming a couple variables which conflicted with #defines from some system headers.

• Addition of btree ordered containers (alternatives to std::map and std::set).
• Fix compilation warnings with msvc.

• support hash for std::pair and std::tuple out of the box
• add a couple of missing includes
• fix compilation issue on msvc when xhash is included.
• fix hash_value() support

• add dump/load feature for hash maps containing std::trivially_copyable values
• throw exception for invalid operator access as standard requires
• allow heterogeneous lookups with std::string_view
• improve serialization when cereal is used.
• fix some compilation warnings.

• add natvis file for friendly hash map visualization in Visual Studio debugger
• add support for Intel C++ compiler

• reorganize headers so that phmap_fwd_decl.h can always be used without compilation errors
• add support to boost's hash_value() method for providing the hash
• fix incorrect return values in phamp::EqualTo causing compilation warnings

Small release, mainly fix warnings and improve internal locking support.

• improve mutex support (with support for boost::shared_mutex and boost::upgrade_mutex)
• add systematic mixing of the hash value provided. This prevents serious degradation of the hash table performance when the hash function provided by the user has poor entropy distribution. The cost in performance is very minimal, and this helps provide reliable performance even with not so good hash functions.
• make the iteration order deterministic by default (can still be made non-deterministic by adding #define PHMAP_NON_DETERMINISTIC 1 before including the header phmap.h (as is done in raw_hash_set_test.cc
• clean and add a couple examples

Fix issue with multiply defined symbols.