Expose parsing #USAGE comments from a script in a string in Rust API

[mdekstrand]

Right now it appears the Rust API exposes 3 ways to parse a Spec:

• parse from a file with parse_file
• parse from a script file with parse_script
• parse from in-memory string (deprecated) with parse

I am attempting to write a program that will embed scripts into a binary (for running scripts with a self-contained interpreter), and this interpreter supports #USAGE comments natively. For this embedded script scenario, I see two solutions:

• Parse the USAGE comments out of the script embedded as a string in the binary.
• Parse the USAGE comments at compile-time and serialize them into the binary (currently blocked by Spec implementing Serialize but not Deserialize).

Either solution would solve my problem; parsing the in-memory #USAGE comments seems simpler to me.

[jdx]

Implemented in #782. It adds Spec::parse_script_str(&str) so embedded scripts can be parsed directly without a temporary file or Spec deserialization. The existing file-based parser retains filename inference and path-aware diagnostics.

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