Support multiple named exports globally

Add multi-root bundle support across the toolchain:
 - `compile`: Accept multiple definition names via `-x NAME` (repeatable or
   comma-separated). Exports all requested definitions as named roots in a
   single bundle. Defaults to "main" when no names are given.
 - `export`: Accept comma-separated hashes in the positional argument and
   multiple `-n`/`names` flags. Exports all resolved roots in one bundle.
 - Server: Add `GET /bundle/roots?n=...&h=...` endpoint that resolves
   multiple stored-term names and/or raw Merkle hashes, returning a single
   bundle containing all of them as roots.
 - Wire: Export `defaultExportNames` helper for generating default export
   names when none are supplied.
 - Drop `cereal` dependency from `tricu.cabal` (no longer used).

tricu.cabal

@@ -42,7 +42,6 @@ executable tricu
     , base16-bytestring
     , base64-bytestring
     , bytestring
-    , cereal
     , cmdargs
     , containers
     , cryptonite
@@ -96,7 +95,6 @@ test-suite tricu-tests
     , base16-bytestring
     , base64-bytestring
     , bytestring
-    , cereal
     , cmdargs
     , containers
     , cryptonite