Convert between different vector sizes on FPGA

[definelicht]

It is sometimes useful to convert between one size vector and another size vector in FPGA codes, for example when converting from 512-bit reads from DRAM to some smaller vector width used in the program.

Right now we only convert between vectorized and non-vectorized.

We should update the codegen to allow arbitrary conversion as long as the data type and total number of elements matches.