Improve type checking errors with structs (#761)

[lgritz]

This is a back-port of a fix that went into master a long time ago, and seems to fix some reported bugs in RB-1.8.

Turns out that in addition to making nicer error printing (its original use, per the commit message), it also fixes a bug wherein use of arraylength() on an array of struct resulted in generated code that would make a confusing error message in oslinfo/oslquery.

Sorry for the confusing diffs; the function is changing a bit, in addition to moving from file to file.

---

Change the implementation of TypeSpec::string(), which returns the text
representation of a TypeSpec, so that it prints the name of the struct
rather than just its numerical designation.

An example of the old behavior, as applied to a type checking error
message:

test.osl:10: error: No matching function call to 'max (struct 1, struct 1)'
    Candidates are:
        int max (int, int)
        float max (float, float)
        ...

And the new behavior:

test.osl:10: error: No matching function call to 'max (struct vector4, struct vector4)'