Add include_file

[dlurton]

Given a file: universe_a.ion, with contents:

// universe_a.ion:
(define toy_lang (domain ... ))

And another file universe_b.ion that resides in a different directory, I should be able to include the universe_a.ion file in universe_b.ion.

We might find a need to make this more sophisticated in the future, but to keep this simple for now, the result will be the same as if the (include_file ...) statement were replaced with the contents of universe_a.ion:

// universe_b.ion:

(include_file "universe_a.ion")

// now domains defined in universe_a.ion are available for permutation

(define permuted_domain 
       (permute_domain toy_lang ...))

The grammar of the include_file statement is:

include_file_stmt :== '(' 'include_file' <string> ')'

Other requirements (not in any order):

• Attempts to include the same file twice should be ignored, thereby preventing cyclic includes.
• After all of the include_file statements in all included files have been processed, the result should be a single type universe with all of the domains from all of the included files, which can then be passed through the remaining processing steps that exist today. No changes to those processing steps are needed.
• Since we're now dealing with multiple files, we must include the path to the file containing the error in any error messages produced by pig. We currently use IonElement's IonLocation meta for this, but it lacks a property for the filename. The filename should either be specified in a different meta, or a new meta should be used that has properties for the source file, line and character offset.
• When error information including the source file, line, column & message is output to the console, the format should be: <relative-path-to-ion-file>:<line>:<column>: <message> This format is understood by certain IDEs such as Visual Studio and IntelliJ (with a plugin) and causes the IDE to to turn the location text in the console output into a clickable hyperlink. (A quality of life improvement.)

[rhinomode]

Without this feature (or discussion of its future), I do not see how this will work in the long term.

Use-case: I want to directly support the some expressions defined by the partiql_ast for my own domain (using permute_domain). Without this feature I copy-paste the entire contents of partiql_ast.ion into my own .ion file.

[dlurton]

Amended the title and description of this issue by replacing the term "import" with "include" to reflect this decision that was made during the review of #74.