src: remove unused ExternString constructor

Remove an unused (and unsafe) constructor.  Unsafe because it doesn't
initialize the data_ field.
src: squelch -Wmaybe-uninitialized warning

The variable isn't actually used uninitialized but g++ 4.8 doesn't know
that.  Set it to NULL to silence the following compiler warning:

    ../src/string_bytes.cc:247:29: warning: 'data' may be used
    uninitialized in this function [-Wmaybe-uninitialized]
         unsigned a = hex2bin(src[i * 2 + 0]);
                                  ^
    ../src/string_bytes.cc:299:15: note: 'data' was declared here
       const char* data;
                   ^
src: fix segfaults, fix 32 bits integer negation

Make calls to v8::Isolate::AdjustAmountOfExternalAllocatedMemory() take
special care when negating 32 bits unsigned types like size_t.

Before this commit, values were negated before they got promoted to
64 bits, meaning that on 32 bits architectures, a value like 42 got
cast to 4294967254 instead of -42.

That in turn made the garbage collector start scavenging like crazy
because it thought the system was out of memory.

That's bad enough but calls to AdjustAmountOfExternalAllocatedMemory()
were made from weak callbacks, i.e. at a time when the garbage collector
was already busy.  It triggered asserts in debug builds and caused
random crashes and memory corruption in release builds.

The behavior in release builds is arguably a V8 bug and should perhaps
be reported upstream.

Partially fixes #7309 but requires further bug fixes to src/smalloc.cc
that I'll address in a follow-up commit.
src: deduplicate CHECK_EQ/CHECK_NE macros

DRY the macros, there is no need to define them twice depending on
whether NDEBUG is defined or not.
src: add CHECK_{GE,GT,LE,LT} macros

Conform to the Google styleguide more and make cpplint happy, add more
CHECK macros.

Preemptively addresses cpplint's readability/check warnings ("Consider
using CHECK_GT instead of CHECK(a > b)".)
src: fix up smalloc weak persistent usage

Fix a regression that was introduced in commit ce04c726 after the
upgrade to V8 3.24.

The new weak persistent handle API no longer gives you the original
persistent but still requires that you clear it inside your weak
callback.

Rearrange the code in src/smalloc.cc to keep track of the persistent
handle with the least amount of pain and try hard to share as much
code as possible between the 'just free it' and 'invoke my callback'
versions of the smalloc API.

Fixes #7309.
src: don't call DecodeWrite() on Buffers

Don't call DecodeWrite() with a Buffer as its argument because it in
turn calls StringBytes::Write() and that method expects a Local<String>.

"Why then does that function take a Local<Value>?" I hear you ask.
Good question but I don't have the answer.  I added a CHECK for good
measure and what do you know, all of a sudden a large number of crypto
tests started failing.

Calling DecodeWrite(BINARY) on a buffer is nonsensical anyway: if you
want the contents of the buffer, just copy out the data, there is no
need to decode it - and that's exactly what this commit does.

Fixes a great many instances of the following run-time error in debug
builds:

    FATAL ERROR: v8::String::Cast() Could not convert to string
src: fix tracing infrastructure after v8 upgrade

Fix up the dtrace/etw/systemtap infrastructure after the V8 upgrade in
commit 1c7bf24.  The win32 changes are untested but can hardly make
things worse because node doesn't build on windows right now.

Fixes #7313 with some luck.
node: Add signature to SET_PROTOTYPE_METHOD.

This prevents segfaults when a native method is reassigned to a
different object (which corrupts `args.This()`).  When unwrapping,
clients should use `args.Holder()` instead of `args.This()`.

Closes #6690.