This patch implements source map support in the parser, the runtime, the
script record, and the stack trace printing.
The library used to parse and use the source maps is gopkg.in/sourcemap.v1.
Unlike earlier versions of this patch, the consumer of otto does not need
parse the source map on their own - it's now handled similarly to parsing
JavaScript content.
To use a source map, the consumer must explicitly parse their source into
a `Script` object with `Otto.CompileWithSourceMap`. The script record
returned from that call will carry source map information with it, and
all location-related functions should reflect the original source
positions.
Previously, it was a publically accessible but invalid value (valueEmpty).
* Deprecate internal use of UndefinedValue(), NullValue(), FalseValue(), TrueValue()
* Guard against Empty, Result, Reference values from escaping the package
