Remove the unused CLI#compile method
Fix description/context of recently added cli specs
Add an output+result catcher helper in cli_spec
Add Kernel#private_instance_methods
Extract the node runner from CLI
Add some naïvely implemented File methods
Add a corelib Process placeholder
Temporary fix for encoding issues (FIXME)
fixup! Extract the node runner from CLI
The Great CLI Overhaul (see details)

- use pluggable runners to execute code (node, server, …)
- make the builder act as the accumulator of compiled sources
- allow user to skip the implicit “opal” require
- user ARGF instead of manually managing the filename (and gain implicit fallback to STDIN)
Cleanup Opal::Server#prefix management
SourceMaps cleanup, added pluggable register

The register defaults to `$OPAL_SOURCE_MAPS = {}`, 
the required api is just #[]= and #[].