Fix dlang#10798 — Reimplement FFT solver to support @nogc nothrow pure @safe

[Ogi-kun]

• FFT solver is reimplemented as a struct called FFT. It either allocates a buffer for a lookup table using malloc, or uses a preallocated buffer. In the former case, the buffer is freed upon destruction. FFT is non-copyable.
• Fft class now is just a wrapper around FFT.
• Convenience functions use FFT instead of Fft. In effect, fft(R, Ret) and inverseFft(R, Ret) can be used in @nogc, nothrow, pure and @safe code.

Though the optimization of the algorithm itself was not the goal of this change, the new version performs slightly better on the larger FFT sizes. Using LDC2 with -O -release flags, on size=64 FFT.compute works about as fast as the old Fft.fft, and on size=32768 it works 6-7 % faster. DMD shows similar results.

[dlang-bot]

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