Fix module caching, closes #107

Use v8::Persistent handle to keep module instances around.

Objects cannot be shared between isolates anyhow, hence moved
modules_loaded map from global V8JSG structure to php_v8js_ctx.

Besides fixes a use-after-free on normalised_module_id.
Merge pull request #108 from stesie/fix-commonjs-module-reuse

Fix module caching, closes #107