Alias declarers internally refactored.

Alias declarers (e.g., :declare_alias_simple()) are now implemented in terms of
existing functionality (e.g., :set_string_to_stdin_help()) rather than manually
inlining such functionality, permitting us to also eliminate all prior globals
declared by @{*-start/*-declare/*-alias} (e.g., ${ZESHY_ALIAS_NAME_TO_HELP}).