Update accuracy of memory usage for `f32`/`f64` in readme

[smu160]

@Shnatsel

Given the results for f32 and f64 memory usage provided by a third party benchmark repo, should we consider updating the "2x lower memory usage" claim in the readme?

To make it easier to read the memory usage data, I took out the other columns to do with timing for both tables:

f32

Sequence length	rustfft memory	phastft memory
8	368 B	32 B
16	624 B	96 B
32	1.152 KB	224 B
64	3.808 KB	480 B
128	6.368 KB	992 B
256	11.48 KB	2.016 KB
512	21.72 KB	4.064 KB
1024	42.20 KB	8.160 KB
2048	83.16 KB	16.35 KB
4096	165.0 KB	32.73 KB
8192	328.9 KB	65.50 KB
16384	656.6 KB	131.0 KB
32768	1.311 MB	262.1 KB
65536	2.622 MB	524.2 KB
131072	5.244 MB	1.048 MB
262144	10.48 MB	2.097 MB
524288	20.97 MB	4.194 MB
1048576	41.94 MB	8.388 MB
2097152	83.88 MB	16.77 MB
4194304	167.7 MB	33.55 MB
8388608	335.5 MB	67.10 MB

f64

Sequence length	rustfft memory	phastft memory
8	320 B	64 B
16	480 B	192 B
32	816 B	448 B
64	3.968 KB	960 B
128	7.040 KB	1.984 KB
256	13.18 KB	4.032 KB
512	25.47 KB	8.128 KB
1024	50.04 KB	16.32 KB
2048	99.20 KB	32.70 KB
4096	197.5 KB	65.47 KB
8192	394.1 KB	131.0 KB
16384	787.3 KB	262.0 KB
32768	1.573 MB	524.2 KB
65536	3.146 MB	1.048 MB
131072	6.292 MB	2.097 MB
262144	12.58 MB	4.194 MB
524288	25.16 MB	8.388 MB
1048576	50.33 MB	16.77 MB
2097152	100.6 MB	33.55 MB
4194304	201.3 MB	67.10 MB
8388608	402.6 MB	134.2 MB

Just eyeballing the results, it seems PhastFT has ≈ 5x lower memory usage for f64 ≈ 3x lower memory usage for f64. Maybe it would be better compute the ratio of rustfft to phastft memory usage for all sequence sizes and then take the average/median/min? Perhaps we should re-run the benchmarks on the original benchmark machine before anything.

Curious to hear your thoughts!