"fix" precision settings

solves Tommy's log problem
Underlying problem:
  map_coefficients is clever: a zero coefficient is mapped
  to zero(target), thus when applying it with
    x->setprecision(x, new)
  the target precision is ignored and the ring precision is used
  instead

src/LocalField/Elem.jl

@@ -693,7 +693,9 @@ function Base.:(^)(a::LocalFieldElem, n::Int)
   e = absolute_ramification_index(parent(a))
   v = valuation(n, prime(parent(a)))
   if v > 0
-    b = setprecision(a.data, precision(a.data)+v)
+    b = setprecision(base_ring(a.data), precision(a.data)+v) do
+            setprecision(a.data, precision(a.data)+v)
+        end
   else
     b = a.data
   end