Add hash function for type AlgebraicNumber

[TZdyrski]

Add a hash function for AlgebraicNumber; it is required that any custom type defining its own == (as AlgebraicNumber does) also define its own hash so that isequal(an1, an2) imples hash(an1) == hash(an2).

This also fixes Base functions like unique; previously, unique([AlgebraicNumber(1), AlgebraicNumber(1)]) produces two elements when it should only produce one (since unique internally uses hash). Now, it correctly returns only a single element.

I only hashed the coefficients (appropriately scaled); this does satisfy the requirement that isequal(an1, an2) imples hash(an1) == hash(an2). However, because hash doesn't use apprx or prec, different AlgebraicNumbers can result in the same hash.

[coveralls]

coverage: 92.531% (+0.4%) from 92.149%
when pulling 9068845 on TZdyrski:master
into b623480 on anj1:master.

[fkastner]

Thank you for this addition!