newSeqUninitialized over-constrained, setLenUninitialized missing

[arnetheduck]

newSeqUninitialized allows creating seq:s that are not initialized, for use as general-purpose buffers.

It is however constrained to SomeNumber, which is unnecessarily restrictive: any type composed only of "trivial" types qualifies for this kind of treatment, similar to how copyMem is constrained to types that supportsCopyMem.

The above functionality is needed to create efficient serializers that preallocate memory then fill it with data from a source (disk/network/etc).

Growing a sequence uninitialized is needed likewise to reuse memory buffers in the above serializers - they do not have access to the capacity of the seq, so there is no way to write an optimal deserializer that reuses the existing capacity of a seq.

[ringabout]

fixed by #22739 and #22767