Adds a variable length binary code library for
coding various symbols for typical use in headers.

The main codes implemented are:
1. Coding a symbol from an n-ary alphabet using a
quasi-uniform code.
2. A bilevel code for coding symbols from an n-ary
alphabet based on a reference value for the symbol
also taken from the same alphabet.
The code has two steps. If the symbol is close to
the reference a shorter code is used, while if it is
farther away a longer code is used.
3. A finite (terminated) subexponential code that codes
a symbol from an n-ary alphabet using subexp parameter k.
4. A finite (terminated) subexponential code that codes
a symbol from an n-ary alphabet using subexp parameter k,
based on a given reference also taken from the same
alphabet. This code essentially reorders the values
before using the same code as 3.

Also adds corresponding encoder side functions to count
the number of bits used.

These codes will be subsequently used for more efficient
encoding of loop-restoration parameters and global motion
parameters.

Change-Id: I28c82b611925c1ab17f544c48c4b1287930764b7