Add at serializer to_slice

[rmja]

This change removes the buffer size generic on the serializer.

For my firmware that uses the simcom grps driver here, the build size is reduced from:

   text    data     bss     dec     hex filename
 318432     120   52888  371440   5aaf0 urcc

to

   text    data     bss     dec     hex filename
 316732     120   52888  369740   5a44c urcc

[MathiasKoch]

Nice! Thanks :+1: