codegen: qualify prelude names in generated service code

[iainmcgin]

Generated service code shares a Rust module with the package's service traits and, in connectrpc-build output, its messages and enums, and each generated module has use super::*;. A proto item named after a prelude item, in the package or a parent package, therefore shadows it. The templates wrote Result, Option, Box, Send, Sync, Clone and Into without a path, so Arrow Flight's message Result fails on every client method:

error[E0107]: struct takes 0 generic arguments but 2 generic arguments were supplied

The templates now write a ::std::… path for everything the stubs do not declare, Some, None and Ok included. Users with checked-in stubs regenerate: every *.__connect.rs file changes in that form only, and the generated API stays the same.

A message, service or sub-package named S or T resolved to a generic parameter of the generated impl blocks, and is now written self::S. The alternative is renaming the parameters to __S and __T, which changes the rustdoc of every generated client and server.

A glob import of such a package still shadows Result in the user's own module; the guide now says to import the module under an alias. The guide's rule for colliding service names said "differ only in case", which Xget and XGet disprove, and is reworded.

Conflicts with #324 in one hunk of codegen.rs and two test files. The second to merge regenerates the checked-in stubs.