Refactoring word expansion

[magicant]

• Move old implementation
• Define new types
  • Phrase: Result of initial-expanding something. An array of any number of fields.
  • Error: Failed expansion. The same as the existing implementation.
  • Result: Result of initial expansion. Either a phrase or an error.
  • Env: Adapter for performing initial-expansion. Extends the normal yash_env::Env with the is_quoted flag and the exit status of the last command substitution.
• Reimplement expansion
• Update callers to use new implementation
• Remove old implementation
• Update documentation

[magicant]

Module structure:

• expansion
  • attr
    • Origin, AttrChar, AttrField
  • phrase
    • Phrase, IntoIter
  • return
    • Return::{Ready, Async}, IntoFuture
  • initial
    • Env, QuoteGuard
    • Expand
    • slice
    • text
    • word
    • command_subst
    • param
  • quote_removal
  • Error, ErrorCause (unchanged)

[magicant]

Considering something like this to optimize word/text unit expansion.

(Update: this does not work because of the limitation found in rust-lang/rust#68117. Instead, I'm thinking of adding a special function in Extend.)