ES6 let and const scoping and minor fixes

• Implement ES6 declaration hoisting
• Fix typo in covers/https to prevented get() from working
• Fix path resolution for nodent.require(...) built-ins
• Fix typos when defining writable Object properties
• Implement "dual-mode" tests that compare sync/async routines directly
• Maintain the position of directives when asynchronizing blocks
• Add dual-mode tests for consts in struct & sloppy mode and scoped try test
• Correct case where await is at the top level in generator mode
• Avoid exception when node is created with no location info
• Include eager.js into runtime.js as source to make istanbul coverage tests work
• Bump acorn-es7-plugin