Improve API

This release adds low-level optimizations, simplify some internal methods and fix few bugs. For more details see change list below.

As of this release, V8 >= 6.0.45 required.

* - BC-breaking or potentially BC-breaking changes

Changes to public API and other important changes which may affect end-user:

• Raise libv8 required version to 6.0.45;
• Add V8\Value::InstanceOf() method;
• Add V8\ObjectValue::SetNativeDataProperty() method;
• Add V8\PropertyFilter, V8\KeyCollectionMode and V8\IndexFilter enums;
• Add property filtering support to V8\ObjectValue::Get{Property,OwnProperty}Names();
• Consistently expose all well-known symbols on V8\SymbolValue;
• Setting boolean props on StackFrame with valid type;
• Remove non-working and non-supporting $attributes argument from V8\StringValue constructor in stubs;
• * Add V8\ConstructorBehavior and support for it in V8\FunctionTemplate constructor;
• * Add signature support in V8\FunctionTemplate constructor via $receiver;
• * Add signature support to Template:SetNativeDataProperty() and ObjectTemplate::SetAccessor() via $receiver;
• * Remove require $isolate parameter from V8\Value::TypeOf();
• * Remove deprecated V8\Context::EstimatedSize() method;
• * Remove deprecated V8\StackTrace\StackTraceOptions;
• * Remove deprecated V8\StackTrace::AsArray();
• * Require Context explicitly in V8\Isolate::ThrowException();
• * Rename V8\Exceptions\GenericException to V8\Exceptions\Exception;
• * Rename V8\Exceptions\AbstractResourceLimitException to V8\Exceptions\ResourceLimitException;
• * Check whether returned context is empty in V8\Isolate\GetEnteredContext() instead of checking InContext();