Optimizes the filterValidXMLChars function by changing the loop variable type from int to uint to eliminate bound checks during compilation, resulting in a ~4% performance improvement.
- Changes loop variable i from int to uint to remove compiler-generated bound checks
- Adjusts type conversions accordingly to maintain correctness
```
goos: linux
goarch: arm64
pkg: miniflux.app/v2/internal/reader/parser
│ old.txt │ new.txt │
│ sec/op │ sec/op vs base │
Parse-8 40.91m ± 3% 39.30m ± 2% -3.94% (p=0.000 n=50)
```
Instead of using bytes.Map which is returning a copy of the provided []byte,
use a custom in-place implementation, as the bytes.Map call is taking around
25% of rss.Parse
io.ReadAll is growing the underlying buffer progressively, while
io.Copy is able to allocate it in one go, which is significantly faster.
io.ReadAll is currently accounting for around 10% of the CPU time of rss.Parse
This will allow to make use of func (*Reader) Seek, instead of re-recreating a
new reader. It's a large commit for a small change, but anything to simply the
reader/buffer/ReadAll/… mess is a step in the right direction I think, and it
should enable more follow-up simplifications.
