Add script compilation and caching

This release adds script compilation and caching. Also more new methods added and some internal logic refactored. See detailed changes below.

As of this release, V8 >= 5.8.168 required.

* - BC-breaking or potentially BC-breaking changes

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

• * Add $is_wasm and $is_module options to V8\ScriptOriginOptions and V8\ScriptOrigin;
• * Remove non-standard V8\Scrip::{getSource,getOrigin};
• * Remove V8\ObjectValue::{Get,Set,Has,Delete,CreateDataProperty}Indexed methods;
• * Remove non-working V8\Context extensions support;
• * Handle V8\IntegerValue sub-types when returning values from V8, fixes #19;
• Add V8\UnboundScript class;
• Add V8\ScriptCompiler\CachedData class;
• Add V8\ScriptCompiler\CompileOptions class;
• Add V8\ScriptCompiler\Source class;
• Add V8\ScriptCompiler;
• Add V8\SymbolValue::GetToPrimitive() method;
• Add V8\Value::IsNullOrUndefinedl() method;
• Add more V8\Value::Is*() methods;
• Add V8\MapObject;
• Add V8\SetObject;
• Fix obj and func templates external memory adjusting.