Janet 0.5.0

Medium-sized update, with a number of improvements including dynamic scoping and some more debugging facilities. From the changelog:

• Fix some bugs with buffers.
• Add trace and untrace to the core library.
• Add string/has-prefix? and string/has-suffix? to string module.
• Add simple debugger to repl that activates on errors or debug signal
• Remove *env* and *doc-width*.
• Add fiber/getenv, fiber/setenv, and dyn, and setdyn.
• Add support for dynamic bindings (via the dyn and setdyn functions).
• Change signatures of some functions like eval which no longer takes
  an optional environment.
• Add printf function
• Make pp configurable with dynamic binding :pretty-format.
• Remove the meta function.
• Add with-dyns for blocks with dynamic bindings assigned.
• Allow leading and trailing newlines in backtick-delimited string (long strings).
  These newlines will not be included in the actual string value.