Because the N macro did not have a project-specific prefix, it clashed
with its namesakes from third-party projects like LLVM. This commit
introduces a namespace-scoped operator ""_n and replaces all invocations
of the N macro with that operator.

The dedicated branch eosio in eosio-wasm-spec-tests contains the changes
that replace the N macro with operator ""_n in the generated-tests
directory.

This commit improves `name operator ""_n` to detect errors in name
constants at compile time by switching to a template implementation.