Further vector refactoring and updates

[Alasdair]

This PR has two commits:

• The first simply removes another usage of undefined in the vector extension, and adds an additional type constraint to allow a runtime assertion to be removed on that function
• The second removes the dec parameters from all the generic vectors in the vector extension (and some other usages, but it's 99% vector), so vector('n, dec, 'a) becomes just vector('n, 'a). For a while now these parameters haven't done anything and are only allowed for backwards compatability.

[github-actions]

Test Results

396 tests  ±0   396 ✅ ±0   0s ⏱️ ±0s
  4 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit 7636462. ± Comparison against base commit 5a841c9.