Support Wasm start section

We should consider to support Wasm start sections. Standard says it’s optional and should be automatically executed when a module is instantiated (https://webassembly.github.io/spec/core/syntax/modules.html#syntax-start). Currently we disabled start sections and we don’t execute a Wasm that contains it. AssemblyScript generates start sections if global variables needs to be instantiated. If we’re to support it we need to add necessary tests to ensure we properly charge for executing it apart from call