Add a Node doc formatter
Use the doc formatter for Node specs
Print the error stack along with the error

This implementation seems to be node-specific, needs a fix.
Use string maps in runtime Hash builders
Fix Hash#inspect with explicit try / finally
Use Hash string map in JSON
Use Hash string map in Native
Go back to the normal formatter
Merge pull request #640 from opal/hash-string-map

Use a separated hash-table for strings/symbols keys