Extract out a source_span package from source_maps.

pkg/source_span/README.md

Index: pkg/source_span/README.md
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+`source_span` is a library for tracking locations in source code. It's designed
+to provide a standard representation for source code locations and spans so that
+disparate packages can easily pass them among one another, and to make it easy
+to generate human-friendly messages associated with a given piece of code.
+
+The most commonly-used class is the package's namesake, `SourceSpan`. It
+represents a span of characters in some source file, and is often attached to an
+object that has been parsed to indicate where it was parsed from. It provides
+access to the text of the span via `SourceSpan.text` and can be used to produce
+human-friendly messages using `SourceSpan.message()`.
+
+When parsing code from a file, `SourceFile` is useful. Not only does it provide
+an efficient means of computing line and column numbers, `SourceFile.span()`
+returns special `FileSpan`s that are able to provide more context for their
+error messages.