Add release notes for 0.42.0rc1

[gmarkall]

Generated with:

python ../numba/maint/gitlog2changelog.py --token="<omitted>" --beginning="v0.41.0" --repo=numba/llvmlite --digits=4

and

python ../numba/maint/gitlog2changelog.py --token="<omitted>" --beginning="v0.41.0" --repo=numba/llvmlite --digits=3

run in the root of the llvmlite repo, checked out next to the Numba repo, and then merging the results, and eliminating duplicate entries that already appeared in the notes for 0.41.0 and 0.41.1