Unsound usages of unsafe implementation from `u8` to `SystemVariables`

[llooFlashooll]

Hi, I am scanning the rtforth in the latest version with my own static analyzer tool.

Unsafe conversion found at: src/memory.rs#L59

    pub fn system_variables(&self) -> &SystemVariables {
        unsafe { &*(self.inner.offset(0) as *const SystemVariables) }
    }

    pub fn system_variables_mut(&mut self) -> &mut SystemVariables {
        unsafe { &mut *(self.inner.offset(0) as *mut SystemVariables) }
    }

This unsound implementation would create a misalignment issues if the type size of u8 is smaller than the type size of SystemVariables.

This would potentially cause undefined behaviors in Rust. If we further manipulate the problematic converted types, it would potentially lead to different consequences such as access out-of-bound. I am reporting this issue for your attention.