Swap out all uses of stdlib for core or "standard library" to core module and standard modules throughout docs & code

[bmillsNV]

Terminology Slang Standard Modules → the modules you get when you download slang Slang Core Module — the module that starts every slang file — thing that can’t opt out of that has all our core types/functions/etc that every module starts with

We don’t want to use stdlib or “library” anywhere. Slang does modules. So we should use modules.