Remove invokable because it is not useful

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Method does not need the is_primitive support, because we have not truffle-ized primitives

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Make method private/protected

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Added assertion for consistency with Double

Signed-off-by: Stefan Marr <git@stefan-marr.de>