cleanup C - Julia BigFloat interop

[MikaelSlevinsky]

C controls mpfr_t memory, Julia controls BigFloat memory.

• Since BigFloats have an extra field, sizeof(BigFloat) == 40, an array is passed as mpfr_t **, and implicitly the only datum in the second level of addressing is dereferenced by [0].
• Since BigFloat is a mutable struct, there is no guarantee that each entry in
  an Array{BigFloat} has a unique pointer. For example, looking at the Limbs,

julia> Id = Matrix{BigFloat}(I, 3, 3)
3×3 Array{BigFloat,2}:
 1.0  0.0  0.0
 0.0  1.0  0.0
 0.0  0.0  1.0

julia> map(x->x.d, Id)
3×3 Array{Ptr{UInt64},2}:
 Ptr{UInt64} @0x000000010f22f958  Ptr{UInt64} @0x000000010f22f918  Ptr{UInt64} @0x000000010f22f918
 Ptr{UInt64} @0x000000010f22f918  Ptr{UInt64} @0x000000010f22f958  Ptr{UInt64} @0x000000010f22f918
 Ptr{UInt64} @0x000000010f22f918  Ptr{UInt64} @0x000000010f22f918  Ptr{UInt64} @0x000000010f22f958

shows that the ones and the zeros all share the same pointers. If a C function
assumes unicity of each datum, then the array must be renewed with a deepcopy.

Before this PR, some of the BigFloat tests were skipped because BigFloat was unsafely converted to mpfr_t before passing to C. I suspect this was unsafe because the garbage collector could free some BigFloat data after the conversion to mpfr_t but before functions were done executing.

[codecov]

Codecov Report

Merging #104 into master will decrease coverage by 0.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
- Coverage   78.65%   78.32%   -0.34%     
==========================================
  Files          12       12              
  Lines        1373     1352      -21     
==========================================
- Hits         1080     1059      -21     
  Misses        293      293              

Impacted Files	Coverage Δ
src/FastTransforms.jl	100.00% <ø> (ø)
src/libfasttransforms.jl	86.55% <100.00%> (-0.91%)	⬇️

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