Immutables 1.1.0

New features

interval_index queries and removals now run through a native C++ engine, yielding large speedups.
Interval endpoints now use the same native query fast path as other structures for character, Date, or POSIXct (in addition to numeric and integer) endpoints.

Corrected the documented return field of the interval pop_* functions to $value
Clarified that peek_point() and the other “first match” interval queries return the match in canonical interval order: smallest start, with insertion/FIFO order breaking ties.
Hardened the pure-R reference tree builder against C-stack overflow when constructing very large structures, by replacing a deep recursion in the bulk builder with iterative node grouping.
Reworking benchmarks vignette in preparation for publication.

CRAN resubmission. Addresses reviewer feedback:
- Added a reference to Hinze and Paterson (2006) doi:10.1017/S0956796805005769 in DESCRIPTION.
- Added missing \value tag for print.flexseq().
- Removed \dontrun{} example wrappers from internal helpers.
Removed the plot() in favor of plot_structure() as part of the developer API.
Added inst/CITATION with a Zenodo DOI for the package.

First CRAN submission.
flexseqs: list-like sequences with push/pop/peek from either end, indexed and named access, insertion, splitting, and concatenation.
priority_queues, ordered_sequences, interval_indexes: finger-tree-backed structures for min/max-by-priority, sorted-by-key, and interval-overlap queries.
Developer API for building custom structures via monoid/measure combinations.
Core operations implemented in C++ via Rcpp, with matching pure-R reference implementations using lambda.r.