More use of feature flags to reduce compile time and binary size

[brassy-endomorph]

In order to facilitate smaller binaries, it would be useful to put some features behind feature flags. For example:

• mod_random could be gated to remove a dependency
• all REPL features/dependencies should be gated away and not included by default
  • installation for the REPL binary would need need to be cargo install --feature repl ketos which seems fine