Allow edition to be specified in rust_repository_set (#319)
Allow edition to be specified in rust_repository_set (#319)

rust_repository_set is a very convenient function to specify all of the
necessary http_repository rules and workspace setup calls.  It however,
forces the edition of rust to be 2015 in the calls of
BUILD_for_rust_toolchain from _load_rust_stdlib.

This change does the following:
 * Adds an edition attr to rust_toolchain_repository (default: 2015)
 * Adds an edition parameter to rust_repository_set (default: None)
 * Adds documentation for rustfmt_version for both
 * Throws in some buildifer fixes of function definitions
bazelbuild/rules_rust,bazelbuild/rules_rust,bazelbuild/rules_rust,bazelbuild/rules_rust,bazelbuild/rules_rust