Span API support

- Remove auto cast arr_real -> arr_cmplx;
- Add `complex(re)` function;
- Add `span` implementation;
- Return `span` from `slice` function for step=1;
- Remove `base_array` reference from `slice`;
- Check same array by pointer range;
- Simplify cast for `cmplx_t` type;
- Add `span` instead `base_array` for some `math` functions;
- Update tests;

Author: vitalsong

benchs/fft.cpp

@@ -86,12 +86,13 @@ BENCHMARK(BM_FFTW3_DOUBLE)
 #endif
 
 static void BM_FFT_DSPLIB(benchmark::State& state) {
+    using namespace dsplib;
     const int n = state.range(0);
-    dsplib::arr_cmplx x = dsplib::randn(n);
-    dsplib::arr_cmplx y = dsplib::fft(x);   ///< update cache
+    auto x = randn(n) + 1i * randn(n);
+    auto y = fft(x);   ///< update cache
     for (auto _ : state) {
         x[0] += 1e-5;
-        y = dsplib::fft(x);
+        y = fft(x);
     }
 }