Simplify titlecase() to unique + match (drop mutable cache)

[yihui]

What

titlecase() previously used a local({}) closure with a mutable cache (list keyed by input value) to memoize tools::toTitleCase() calls across invocations (issue #129). This replaces it with a plain unique + match:

convert <- function(text) {
  if (lower) text <- tolower(text)
  u <- unique(text)
  tools::toTitleCase(u)[match(text, u)]
}

Why

• The mutable cache added state and complexity for no benefit. toTitleCase() was already only ever called on distinct values within a single call; the cross-call cache only helped if the same strings recurred across separate titlecase() invocations, which is not the hot path.
• Benchmarked over 20 runs on 200k values: the simple version (1.95s) is faster than the warm-cache memoized version (2.26s), because building/reading the cache list costs more than it saves.
• Output is byte-identical: verified on character vectors, factors (levels preserved), empty input, and the existing lower = TRUE/FALSE paths.

Testing

• tests/testthat/test-ae_listing.R (covers titlecase()/propercase()): 5/5 pass.
• Full suite: 0 failures, 21 tests.

This PR is independent of the bug-fix PR (#162) and the vectorization PR — it touches only R/ae_listing.R.