Inline basicMul from math/big for small products.
Inline basicMul from math/big for small products.

Makes FFT more interesting at small sizes.

benchmark                old ns/op    new ns/op    delta
BenchmarkMulFFT_1kb          29577        25226  -14.71%
BenchmarkMulFFT_10kb        293288       259680  -11.46%
BenchmarkMulFFT_50kb       1678775      1538970   -8.33%
BenchmarkMulFFT_100kb      2622835      2437556   -7.06%

remyoudompheng/bigfft