Refactor object_usage_linter() to use xml_nodes_to_lint()

[AshesITR]

fixes #1284

• remove get_function_assignments()
• use more robust location matching via XPath
• clean up (CC is 8 now)
• rewrite get_content() to accept xml_node as info input and use line info as well
• move usage parse adjustments to parse_check_usage()
• improve get_assignment_symbols() by using get_r_string() instead of hand-rolled unescaping

[AshesITR]

Need a news bullet for improved location info in #834 once merged.

Not happy with the performance atm. Looking into it.

[AshesITR]

Improvements halves the time, but still at 14.5s vs. 5.7s for main on tools/QC.R.
Willing to sacrifice some performance for the improved location info, but not that much.

[AshesITR]

Okay, turns out xml2::xml_find_num is way slower than xml2::xml_attr, so I wrote xp_find_location(xml, xpath) which special cases the two important cases number(./@col1) and number(./@col2).

This probably speeds up all linters using the default location info when they generate many lints.