This commit adds a `Loader` type, which can be used to load multiple
source strings. This was done to support the work on modules, but
coincidentally enabled consolidating errors, since now `Config::run`
can take a `&Loader`, and in the event of an error, return and `Error`
that borrows from loaded strings. Multiple error types have been
consolidated, and a bunch of ad-hoc error printing was removed.
- Combine and simplify string and backtick lexing.
- Allow newlines in strings and backticks.
- Add triple-delimited indented strings and backticks. Common indented literal non-blank line leading whitespace is stripped.
- If a literal newline is escaped, it will be suppressed.
- Backticks starting with `#!` are reserved for a future upgrade.
Unify lexing of backticks, cooked strings, and raw strings. Also allow
newlines in backticks and cooked strings, since I can't think of a reason
not to.
