Drop Pair interface

[termoshtt]

Pair interface

Basically Pair interface has been introduced to use safe interface in Rust. Original interface of FFTW assumes plan.execute() form which modify arrays saved in plan, but this interface is not safe in Rust.
However, we has obtained "new array execution" interface in #44, which allow safe interface s.t. plan.execute(&mut from, &mut to).

• This PR drops old Pair-based interface.
• ndarray-based interface is postponed to future PR...

[codecov]

Codecov Report

Merging #50 into master will increase coverage by 4.78%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   89.43%   94.21%   +4.78%     
==========================================
  Files          14        6       -8     
  Lines         511      173     -338     
==========================================
- Hits          457      163     -294     
+ Misses         54       10      -44

Impacted Files	Coverage Δ
src/error.rs	0% <0%> (ø)	⬆️
tests/r2c.rs	100% <100%> (ø)	⬆️
src/array.rs	100% <100%> (ø)	⬆️
tests/c2c.rs	100% <100%> (ø)	⬆️
src/plan.rs	94.91% <96%> (ø)	⬆️

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