- Rename fc.Indent() → fc.Indented() for indenting code generated in a
    callback.
  - Add fc.Indentation() to generate the indentation sequence itself.

(cherry picked from commit f86fde6)

 - Document semantics of the `functionContext` fields (to the best of my
   knowledge, at least).
 - Move `signatureTypes` to the `typesutil.Signature` and add unit
   tests.
 - Make use of the `typesutil.Signature` helper type for more
   self-explanatory condition checks in the compiler code.

(based on commit 161d21d)

These functions provide mappings from Go entities (variables, types,
functions, etc.) to JS identifiers assigned to them. This commit adds
documentation comments and renames a couple of methods to better match
their role.

(based on commit 4767913)

This is the first step in reducing complexity of the compiler.Compile
function.

The new `sources` type represents all inputs into the package
compilation and simplifies extracting useful information out of them. It
is designed to have little business logic of its own and serves as a
convenient bridge to other packages like go/types or astrewrite.

The ErrorList type is extended with utility methods that reduce
verbosity of the calling code.

(cherry picked from commit cb5cf57)