optional: Import tl's optional type

This is directly adapted from https://github.com/TartanLlama/optional.
This class is tested for C++11 and on GCC 4.8, which is our target mimimum
GCC version.

This optional class provides multiple advantages over our existing
implementation of Optional<T>:

- Easier API (no static constructors)
- Better method chaining (and_then, map...)
- Harsher semantics (no assign-through on Optional<T&>)

Over the std::optional type present in C++'s standard library, this type offers:

- Optional references without going through std::reference_wrapper
- Better method chaining

For these reasons, I am in favor of importing it directly rather than
adapt C++14's std::optional type to work with C++11 and GCC 4.8.

gcc/rust/ChangeLog:

	* util/optional.h: New file.

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