rust: add support for Intel PT regions (#140)

* rust: add `__itt*` functions in generated bindings

This change adds some bindgen-generated functions, and not just
variables, to the generated bindings. This should resolve #139 by making
the `pt_region_*` functions at least visibile in the `ittapi-sys` crate,
making it possible to add higher-level Rust versions.

We cannot generate bindings for all declared functions because these
cause link errors: they are only present in the dynamic library (`*.so`)
but not in the static library (`libittnotify.a`) that the `ittapi-sys`
crate links to. So, for these dynamically-provided functions we must
just retain the `*_ptr__3_0` data symbols and wait for them to be
resolved to functions pointers at runtime.

* rust: remove unused code

* rust: document static/dynamic linking in `README.md`

It is easy to forget how the `ittapi` system of static + dynamic linking
works (and why!). To answer the underlying question of issues like #139,
this change adds some `README.md` documentation explaining why we can't
just include all function symbols in the generated bindings.

* rust: add `Region` for marking Intel PT regions

In #139, @codecnotsupported pointed out a need for access to
`__itt_mark_pt_region_begin` and `__itt_mark_pt_region_end` for
fine-grained VTune analysis using Intel PT. This change adds a
high-level `Region` structure for easy access to these now-available
functions. Closes #139.