http: avoid some copies

[arnetheduck]

While waiting for:

• Move values out of Future on await #601
• sink rewriting of parameters in {.async.} #578
• Missing implicit move in assignment nim-lang/Nim#25057

In particular, the move-on-return are ugly but needed for #639 to do its job well

[arnetheduck]

Pre:

| Small/small    |    0.059s |   1000 | 16830.970 |    0.031 MB |    0.016 MB |    0.514 MB/s |    0.273 MB/s |
| Medium/small   |    0.783s |   1000 | 1276.412 | 1000.000 MB |    0.021 MB | 1276.412 MB/s |    0.027 MB/s |
| Small/Medium   |    0.993s |   1000 | 1007.081 |    0.031 MB | 1000.000 MB |    0.031 MB/s | 1007.081 MB/s |
| Medium/Medium  |    1.475s |   1000 |  677.819 | 1000.000 MB | 1000.000 MB |  677.819 MB/s |  677.819 MB/s |

Post:

| Small/small    |    0.061s |   1000 | 16490.837 |    0.031 MB |    0.016 MB |    0.503 MB/s |    0.267 MB/s |
| Medium/small   |    0.760s |   1000 | 1315.258 | 1000.000 MB |    0.021 MB | 1315.258 MB/s |    0.028 MB/s |
| Small/Medium   |    0.919s |   1000 | 1088.442 |    0.031 MB | 1000.000 MB |    0.033 MB/s | 1088.442 MB/s |
| Medium/Medium  |    1.353s |   1000 |  738.962 | 1000.000 MB | 1000.000 MB |  738.962 MB/s |  738.962 MB/s |

[arnetheduck]

Avoiding nim-lang/Nim#25063:

| Medium/Medium  |    1.093s |   1000 |  914.615 | 1000.000 MB | 1000.000 MB |  914.615 MB/s |  914.615 MB/s |

removed @ change from this branch (it needs more work)