Version 0.1.0

• Zval::reference() returns a reference instead of a dereferenced pointer.
• Added ZendHashTable::iter() - note this is changing in a future version.
• ClassBuilder::extends() now takes a reference rather than a pointer to match the
  return type of ClassEntry::exception().
• ClassEntry::build() now returns a reference - same reason as above.
• Improve library 'safety' by removing unwrap calls:
  • .build() returns Result on FunctionBuilder, ClassBuilder and ModuleBuilder.
  • .property() and .constant() return Result on ClassBuilder.
  • .register_constant() returns Result.
  • .try_call() on callables now return Result rather than Option.
  • throw() and throw_with_code() now returns Result.
  • new() and new_interned() on ZendString now returns a Result.
  • For ZendHashTable:
    • insert(), insert_at_index() now returns a Result<HashTableInsertResult>, where Err failed,
      Ok(Ok) inserts successfully without overwrite, and Ok(OkWithOverwrite(&Zval)) inserts successfully
      with overwrite.
    • push() now returns a Result.
    • Converting from a Vec or HashMap to a ZendHashTable is fallible, so it now implementes TryFrom as
      opposed to From.
  • For Zval:
    • set_string() now returns a Result, and takes a second parameter (persistent).
    • set_persistent_string() has now been removed in favour of set_string().
    • set_interned_string() also returns a Result.
    • set_array() now only takes a ZendHashTable, you must convert your Vec or HashMap
      by calling try_into() and handling the error.