- New rule 'xgo_binary' takes a list of go_binary dependencies and an
  'os_arch' string attribute.  Build output 'foo' is namespaced in
  bazel-bin/PACKAGE_NAME/_xgo/GOOS_GOARCH/foo.  Cross-compilation is
  achieved via a parameterized aspect implementation that propogates
  down the 'deps' attribute; platform target is informed by the
  'os_arch' attribute whose values are enumerated in
  //go/private/goos_goarch.bzl.

- New rule 'go_root' captures the absolute path of GOROOT at the time
  of toolchain installation.  This rule is only used internally.  For
  some reason cross-compilation requires an absolute path to GOROOT.

- Repository rule 'go_repositories' takes an optional environment
  variable BAZEL_GOROOT to use local go toolchain if desired.

- Partitions def.bzl file into individual bzl files in go/private
  where possible.  Each file implements a single rule.  These are
  loaded by def.bzl, so end-user is unaffected.

- Reorganization of def.bzl into sections: constants, utility
  functions, action functions, aspects, and rules.

- Reengineered go_library_impl and go_binary_impl into functions
  go_library_build and go_binary_build that act as common entrypoints
  for the reworked *_impl functions as well the new aspect
  implementations _xgo_*_impl.  These functions are agnostic to the
  layout of the ctx argument (different for normal rules vs. aspect
  rules).  The parameter lists are longer but declare their
  requirements more explicitly.

- Expanded documentation of most functions with skydoc docstrings and
  type annotations.

- Cross-compilation of cgo targets is not supported.  Assembly files
  are permitted but must be consistent with the target architecture.