util: add isPrimitive check
smalloc: fix assertion fails/segfault

* Numeric values passed to alloc were converted to int32, not uint32
  before the range check, which allows wrap around on ToUint32. This
  would cause massive malloc calls and v8 fatal errors.
* dispose would not check if value was an Object, causing segfault if a
  Primitive was passed.
* kMaxLength was not enumerable.
smalloc: remove double checks

Now that values are checks in JS, no need for them to be checked in C++.
src: remove non-isolate PersistentToLocal()

There is no need for it and it's a tiny bit slower than the version of
PersistentToLocal() that takes a v8::Isolate* as its first argument.
process: set key properties as ReadOnly
src: remove non-isolate PersistentToLocal(), v2

Commit 78d9094 updated src/*.cc to use the version of PersistentToLocal
that takes a v8::Isolate* as its first argument. This commit removes
the non-isolate version.
crypto: fix another over-run in bio

When doing `FreeEmpty`, `NodeBIO` skips pre-allocated `head_` buffer.
However this might lead to double-freeing buffers since in `~NodeBIO()`
we're starting deallocation from `head_` buffer.
src: use PersistentToLocal() in a few more places

Update a few more `Local<T>::New(isolate, persistent)` call sites to
`PersistentToLocal(isolate, persistent)` - the latter has a fast path
for non-weak persistent references.
src: use static_cast for void-to-T casts

Code cleanup, replace a few uses of reinterpret_cast<T*>(void_ptr) with
static_cast<T*>(void_ptr).
tls: fix handling of `SNICallback` server option

It shouldn't ignore it!

There're two possibile cases, which should be handled properly:

1. Having a default `SNICallback` which is using contexts, added with
  `server.addContext(...)` routine
2. Having a custom `SNICallback`.

In first case we may want to opt-out setting `.onsniselect` method (and
thus save some CPU time), if there're no contexts added. But, if custom
`SNICallback` is used, `.onsniselect` should always be set, because
server contexts don't affect it.
v8: fix openbsd build

This is [1] applied ahead of time. Summary:

    OpenBSD doesn't have <ucontext.h>. ucontext_t lives in <signal.h>
    and is a typedef for struct sigcontext. There is no uc_mcontext.

[1] https://codereview.chromium.org/21705003/