Support up/down cast

[kenz-gelsoft]

• upcasting is safe and should succeed.
• downcasting can fail, but should be safe.
  • try_ somthing naming is desired.

[kenz-gelsoft]

upcasting may be expressed with AsRef< Base>

https://doc.rust-lang.org/std/convert/trait.AsRef.html

[kenz-gelsoft]

downcast could be expressed as:

impl TryFrom<AsRef<Base>> for &Derived or so?

[kenz-gelsoft]

upcasting may be expressed with AsRef< Base>

https://doc.rust-lang.org/std/convert/trait.AsRef.html

• Current wx wrapper impl has no pointer (to pointer) for multiple-inheritance base class (should be pointed base ptr + vtable offset).
• So couldn't be expressed as AsRef<Base> successfully (at least, for me, yet...).

[kenz-gelsoft]

Try to represent base/derived classes with From/TryFrom traits.

[kenz-gelsoft]

Use these scenarios as testbed:

• upcasting: https://github.com/kenz-gelsoft/wxRust2/pull/106/files#diff-fc00266febcc56dd196fb29ded80babf3ba5d080a66a0174ec7b1075c6280989R556
• downcasting: https://github.com/kenz-gelsoft/wxRust2/pull/106/files#diff-fc00266febcc56dd196fb29ded80babf3ba5d080a66a0174ec7b1075c6280989R533