[FIRRTL] Add create vector/bundle ops

As [discussed](https://github.com/llvm/circt/pull/1304#discussion_r659352639) in #1304, constructing a vector or bundle currently requires a temporary `firrtl.wire`, with subfield/subindex ops and connects. This makes it generally difficult to find out if a vector/bundle is a constant, among other things.

It might make sense to introduce `create_vector` and `create_bundle` ops, similar to HW's `create_array` and `create_struct`, to generaten aggregate value from fields. It might also make sense to have a `create_vector_uniform` (after LLHD's `array_uniform`) which constructs a vector where all elements are the same.

Start with the discussion in pull request #1304 and compare HW's create_array/create_struct with LLHD's array_uniform. Clarify the semantics and scope of create_vector, create_bundle, and any uniform variant; the work is done when the agreed aggregate construction operations are implemented and their behavior is verified.

Written by the indexing model from the issue text.