Alloc + no_std: part 2

[soywod]

This part 2 is a bit bigger, but changes are small and still reviewable in my opinion. I can split if you want.

• Prevented serde to enable std, and in a more generic way made sure that serde and std are 2 separated things
• Bumped octseq to 0.6.1 to benefit from Fix no_std and alloc. octseq#68
• Replaced std gates by alloc ones, when applicable
• Used core and alloc instead of std, when applicable
• Fixed a typo in src/base/header.rs where std gate was used instead of rand

I mention that Claude Code helped me to find references and test. I reviewed every single line.

Refs: #572 #641 NLnetLabs/octseq#68