don't store alignments in `TypeLayout`

[RayMarch]

in #19 we noticed that TypeLayout should maybe not contain alignment information at all
see https://github.com/RayMarch/shame/pull/19/files#r2166782448
instead it should only store relative offsets of fields, sizes and array strides.
Alignments should be inferred from the chosen Repr and the TypeLayoutSemantics to allow composition still.