Reformat max_threat()

Helper function should just know how to find the
biggest piece type in a bitboard. All the threat
logic and data shoud be in evaluate_threats().

This nicely separates the scope of the two functions
in a more consistent way and simplifies the code.

No functional change.
Retire to_char() helpers

Remove some useless wrappers and make
the conversion explicit and starightforward.

No functional change.
Rename ucioption.h to uci.h

We are going to add all UCI related
functions here, so first rename it
to a more proper name.

No functional change.
Retire notation.h

And move the few remaining content
under UCI namespace where they belong.

No functional change.
Final UCI helpers renaming

To reflect new changes, specifically that
now are all under UCI namespace.

No functional change.
Retire notation.cpp

Now we can finally retire notation.cpp
and move UCI helpers under uci.cpp

No functional change.
Fix an obscure gcc warning

warning: narrowing conversion from ‘int’ to ‘char’ inside { }
is ill-formed in C++11 [-Wnarrowing]

When pedantic meets esoteric!

No functional change.