Micro-optimize command-to-RESP

[zuiderkwast]

This avoids a few nested lists, thus a few allocations.

[bjosv]

We had some figures for this change by running them in a test run, do you have them? Just for the record.

[zuiderkwast]

It's hard to get stable figures, but here is my best try.

Each test run encodes a large pipeline of commands (7 commands of 4821 bytes in
total) one million times. The times are nanoseconds per encode.

Run	With this PR	main branch	Improvement %
1	7319	8100	9.64
2	6962	8221	15.31
3	8988	8400	-7.00
4	8144	8091	-0.65
5	8279	8094	-2.28
6	8092	8481	4.58
7	7397	7892	6.27
8	7039	7832	10.12
9	7464	7645	2.36
10	8105	8114	0.11
Avg	7778.9	8087	3.81

It's easier to motivate it theoretically.

Each nested list which is replaced by its elements inlined in the outer list saves one cons cell, i.e. one allocation of 2 words (16 bytes). So with fewer allocations, it's also less work for the garbage collector.

Example: [... , $\r, $\n, ...] instead of [... , "\r\n" , ... ] saves one cons cell.