Improve printing for `AbsSimpleNumField`

[fingolfin]

From oscar-system/Oscar.jl#2878 (comment):

julia> K,a = cyclotomic_field(3, "a")
(Cyclotomic field of order 3, a)

julia> K
Number field with defining polynomial _$^2 + _$ + 1
  over rational field

Note how the "restricted" printing we see at first says "Cyclotomic field" but then the supposedly "detailed" printing omits this.

Secondly, the generator is named a, I would expect that to be part of the printing?

And finally the $_ may be technically best etc. but I think a regular user would find it easier if we just called that variable x? Just like QQBar does.

How about e.g.

julia> K
Cyclotomic field with generator a and defining polynomial x^2 + x + 1
  over rational field