Add as = "Rle" return type to read_bigwig() (#18)

[jayhesselberth]

Summary

Adds read_bigwig(..., as = "Rle"), requested by @jmw86069 in #18, for
per-base coverage queries (e.g. over exons) as a lighter-weight
rtracklayer replacement.

• Returns a per-base run-length-encoded vector spanning the queried range:
  a single Rle for one chromosome, or a named RleList for several.
• Expanded length equals end - start when both are supplied, otherwise the
  data extent of each chromosome. bigWig coords are 0-based half-open, so
  element i is genomic position start + i - 1.
• Uncovered bases are set to fill (default 0, the coverage convention;
  pass fill = NA to mark them missing).

Implementation notes

The conversion lives in R (as_rle() / runs_to_rle() in R/read.r). The
C++ layer (read_bigwig_cpp) already emits run-length data, and an Rle's
payload is just (values, lengths), so only compact runs cross the
C++/R boundary — building the S4 object stays in R alongside the
S4Vectors/IRanges classes that define it. No C++ changes.

Adds S4Vectors to Imports (for Rle) and imports RleList from
IRanges.

Tests

New local (non-network) tests cover windowed length, interior gap filling,
fill = NA, whole-chromosome extent, and the multi-chromosome RleList.
Full suite passes locally (68 tests). Leaving the full R CMD check to CI.

Note

Builds on the remote Range-request fix (#18, already on main) — per-exon
remote queries would crash without it.