Allow coercion and conversion from AcbFieldElem and ComplexFieldElem to Float64 and ComplexF64

[jamesnohilly]

This PR adds conversion functionality for Float64 and type-based conversion for ComplexF64 for AcbFieldElem and ComplexFieldElem. It follows the same structure as ArbFieldElem by splitting the conversion code into their types and using this in the appropriate convert.

This PR references oscar-system/Oscar.jl#4531.

[fingolfin]

@thofma so we have this convert method here because it existed before... We can remove it, but technically this is breaking. Thoughts? (I am fine with breaking ; or leaving it in; as long as we agree)

[fingolfin]

@thofma This convert method was added "for symmetry". But of course we can also remove it and only keep the convert to ComplexF64 (to retain compatibility), or drop both...

[thofma]

I guess we can keep the convert methods, since we already have a bunch for them.

[fingolfin]

We really should have constants for this. This is the declaration in C:

typedef enum
{
    ARF_RND_DOWN  = 0,
    ARF_RND_UP    = 1,
    ARF_RND_FLOOR = 2,
    ARF_RND_CEIL  = 3,
    ARF_RND_NEAR  = 4
}
arf_rnd_t;

We should have something similar, say

const ARF_RND_DOWN  = 0
const ARF_RND_UP    = 1
const ARF_RND_FLOOR = 2
const ARF_RND_CEIL  = 3
const ARF_RND_NEAR  = 4

somewhere in the code (for now we could create this manually, on the long run this could be created/updated from the FLINT code by a script @lgoettgens is working on).

@jamesnohilly please make a follow-up PR to add these constants somewhere (perhaps in src/arb/ArbTypes.jl for now), and use them at least in the code you are adding here (and possibly other places calling arf_get_d)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.33%. Comparing base (b1cd279) to head (ce80066).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2018      +/-   ##
==========================================
+ Coverage   88.31%   88.33%   +0.02%     
==========================================
  Files          98       98              
  Lines       36181    36203      +22     
==========================================
+ Hits        31953    31981      +28     
+ Misses       4228     4222       -6     

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