Improved scripts

Improve built-in scripts:

• Add script to create AWS user role, remove instructions for manual removal from readme
• Add scripts to delete ECR repo, Lambda function, user role from AWS
• Use more precise language around AWS URIs vs ARNs
• Check for the existence of AWS resources in scripts then react accordingly (eg, leave as is, delete & re-create)
• Reduce unnecessary debug output from running scripts

Incorporate PackageCompiler.jl

Restructure code to allow use of PackageCompiler.jl:

• Move Lambda Runtime into separate repo (https://github.com/harris-chris/julia-lambda-runtime)
• Add -p option to have docker image precompiled