tamer: Introduce context into XirReadermain

tamec and tameld will now both introduce a `Context` to XIR, which will use
it to create spans.

Here's an example of an error, now that it's all working well together:

  $ target/release/tameld --emit xmle -o /dev/null path/to/package.xmlo
  error: invalid preproc:sym/@dim `9` at [/../path/to/package.xmlo offset 1175451-1175452]

A future task will make this human-readable by producing line and column
numbers, and perhaps even a snippet (if not now, then eventually).

It's exciting to see this coming together finally.

DEV-10934

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