Add ByteString.endsWith and expand startsWith test coverage

[pjfanning]

ByteString had startsWith(Array[Byte]) but no endsWith equivalent, and the existing startsWith tests lacked edge-case coverage.

New API

endsWith(bytes: Array[Byte]): Boolean added to the ByteString base class, mirroring startsWith semantics:

ByteString("abcdefghijklmnopqrstuvwxyz").endsWith("uvwxyz".getBytes) // true
ByteString("abcdefghijklmnopqrstuvwxyz").endsWith("12345".getBytes)  // false
ByteString.empty.endsWith(Array.emptyByteArray)                      // true

Test improvements

• startsWith: added coverage for empty byte array (always true), exact-length match, bytes longer than the string (false), ByteString1C, and empty ByteString
• endsWith: full suite covering ByteString1, ByteString1C, ByteStrings (multi-segment, including cross-boundary suffix), ByteString1 with internal offset, empty byte array, exact match, oversized input, and empty ByteString

[pjfanning]

I intend to look at adding a set of JMH benchmarks.
The existing impl inherited from Seq class involves creating iterators and doing drops and I would expect that this comes with a performance penalty for ByteString.

[pjfanning]

With Changes
[info] ByteString_startEnd_Benchmark.bs_endsWith         thrpt    3  276333055.676 ±  61641905.180  ops/s
[info] ByteString_startEnd_Benchmark.bs_endsWithBytes    thrpt    3  114538811.656 ± 167288641.049  ops/s
[info] ByteString_startEnd_Benchmark.bs_startsWith       thrpt    3  124530515.986 ±  36356815.658  ops/s
[info] ByteString_startEnd_Benchmark.bs_startsWithBytes  thrpt    3   71968560.978 ± 179926032.403  ops/s
[info] ByteString_startEnd_Benchmark.bss_endsWith        thrpt    3   12830794.623 ±   6215170.619  ops/s
[info] ByteString_startEnd_Benchmark.bss_startsWith      thrpt    3   86003737.814 ± 397051764.052  ops/s

Without Changes
[info] ByteString_startEnd_Benchmark.bs_endsWith         thrpt    3   52301743.336 ± 249800803.546  ops/s
[info] ByteString_startEnd_Benchmark.bs_endsWithBytes    thrpt    3   57212290.541 ±  36873077.965  ops/s
[info] ByteString_startEnd_Benchmark.bs_startsWith       thrpt    3    1865819.744 ±   8946244.499  ops/s
[info] ByteString_startEnd_Benchmark.bs_startsWithBytes  thrpt    3   47994920.794 ± 195723663.984  ops/s
[info] ByteString_startEnd_Benchmark.bss_endsWith        thrpt    3     295568.162 ±   1047083.734  ops/s
[info] ByteString_startEnd_Benchmark.bss_startsWith      thrpt    3    2736217.171 ±  10193061.120  ops/s