Implement basic span error reporting

[mitsuhiko]

Some operations now report the spans of where the error happened.

Example error in debug format:

Error {
    kind: TooManyArguments,
    name: "err_bad_test_arguments.txt",
    line: 2,
}

---------------------------- Template Source -----------------------------
   1 | {% for item in seq %}
   2 >   {% if item is even(42) %}
     i                 ^^^^^^^^ too many arguments
   3 |     {{ item }}
   4 |   {% endif %}
   5 | {% endfor %}
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