Fix #824

Fixes #762

Fix #744

Remove mutable which causes reentrant issue

Change-Id: Ia96ddf5746f1c18968e9e086f17fe4a24b8480d7

Fix inconsistent calling of template functions in PutN in stream.h. When
used with a GenericStringBuffer<<UTF8>, MemoryPoolAllocator>, PutN would call
PutReserve from stream.h, and PutUnsafe from stringbuffer.h. This
resulted in bytes being added to the buffer without allocating space.

This was not an issue when used with the default memory allocator,
because in this case the specialized PutN is used from stringbuffer.h.

GCC 6.x doesn't yet support this warning flag, as reported by
@ragnar-ouchterlony.

* document.h
  * suppress -Wterminate on GCC 6.x and later
  * simplify warning handling
* schema.h
  * drop RAPIDJSON_NOEXCEPT from GenericSchemaDocument constructor
    (calls RAPIDJSON_NEW anyway)
  * simplify warning handling
    (avoids RAPIDJSON_POP mismatch on Clang)
* encodingtest.cpp, istreamwrappertest.cpp
  * work around -Wdangling-else
* readertest.cpp
  * suppress -Wdangling-else

note that std::numeric_limits<uint64_t>::max() and
std::numeric_limits<int64_t>::max() aren't exactly representable in a
double, so we need to be strictly less to be definitely lossless

UBSAN gave during Value.IsLosslessDouble test:
include/rapidjson/document.h:955:42: runtime error: value 1.84467e+19 is outside the range of representable values of type 'unsigned long'

UBSAN gave:
runtime error: division by zero

maybe these tests should just be deleted?

UBSAN gave:
runtime error: signed integer overflow: -9223372036854775808 - 1 cannot be represented in type 'long'
runtime error: signed integer overflow: -9223372036854775808 - 2 cannot be represented in type 'long'

UBSAN gave for test/unittest/itoatest.cpp:87:
runtime error: signed integer overflow: 4611686018427387904 * 2 cannot be represented in type 'long'

UBSAN gave "runtime error: index 13 out of bounds for type 'const uint32_t [10]'"

specifically, "expression with side effects has no effect in an unevaluated context"

clang advises: "note: silence by adding parentheses to mark code as explicitly dead"

This adds kWriteNanAndInfFlag to Writer to allow writing of nan,
inf and -inf doubles as "NaN", "Infinity" and "-Infinity",
respectively, and kParseNanAndInfFlag to Reader to allow parsing
of "NaN", "Inf", "Infinity", "-Inf" and "-Infinity". This is part
of issue #36, adding optional support for relaxed JSON syntax.

Due to dereferencing a pointer which may be invalidated