Optimize local fields a bit

[joschmitt]

This should make some things a bit faster in the local fields.
I had timings where _p_adic_regulator was twice as fast as before, but the timings differ largely accross julia sessions.
This closes #1537 as far as I am concerned.

Not all changes here are "trivial", so someone should have a look (once the CI is green).

[joschmitt]

This was incorrect/non-optimal before:

julia> F, _ = cyclotomic_field(3);

julia> OF = maximal_order(F);

julia> K, toK = completion(F, 2*OF);

julia> precision(K)
64

julia> a = K(2)^100
2^100 + O(2^165)

julia> a += one(K, precision = precision(a))
2^0 + 2^100 + O(2^165)

julia> isone(a)
true

[codecov]

Codecov Report

Attention: Patch coverage is 95.34884% with 4 lines in your changes missing coverage. Please review.

Project coverage is 75.59%. Comparing base (b951296) to head (f5cf2b2).

Files	Patch %	Lines
src/LocalField/Elem.jl	94.36%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1567      +/-   ##
==========================================
- Coverage   75.66%   75.59%   -0.08%     
==========================================
  Files         357      357              
  Lines      113542   113549       +7     
==========================================
- Hits        85915    85840      -75     
- Misses      27627    27709      +82     

Files	Coverage Δ
src/LocalField/Completions.jl	96.56% <100.00%> (ø)
src/LocalField/LocalField.jl	76.57% <100.00%> (+0.58%)	⬆️
src/LocalField/Types.jl	100.00% <100.00%> (ø)
src/LocalField/Elem.jl	82.52% <94.36%> (-0.40%)	⬇️

... and 29 files with indirect coverage changes