Create a generic BTreeG for Go 1.18 and beyond.

[gconnell]

With the introduction of generics in Go 1.18, we have the opportunity
to use them for our btrees. However, we want to make sure that our API
remains compatible with past uses. So, we introduce a new BTreeG which
is a generic API. We then utilize it to recreate the original BTree
of Item interfaces.

Go compilers <1.18 will compile btree.go, containing just the original
BTree of Items. Go compilers >=1.18 instead compile btree_generic.go
which creates the BTreeG generic and uses it to make a BTree of Items
with the same API as that exposed in btree.go.

One major difference between the APIs of BTree and BTreeG is that, for
BTree, we could return a nil Item to connote "nothing was found". For
generics, users may often want to store the zero value, so we can't
treat the zero value as "missing". For that reason, you'll see these
differences in APIs:

  func (t *Btree)     Get(item Item) Item      { ... }
  func (t *BtreeG[T]) Get(item T)    (T, bool) { ... }

Note that we now return a second boolean return value, which is true
if the value was found and false otherwise.

Using the generic for base types like int greatly speeds up processing,
since it removes the need for the Item interface to be
created/malloc'd, and it allows more contiguous storage of values within
the BTree's nodes themselves.

As expected, all G (generic) ops are notably faster than their associated
Int Item ops, because of the removal of interface overhead. Untested here,
but they should also be far less memory-fragmented, since values are
stored within the node item arrays rather than pointed to by interfaces
within those arrays.

BenchmarkInsertG-4 3014830 355.2 ns/op
BenchmarkInsert-4 2296561 639.9 ns/op

BenchmarkSeekG-4 5478997 218.6 ns/op
BenchmarkSeek-4 2880756 396.0 ns/op

BenchmarkDeleteInsertG-4 1720306 653.0 ns/op
BenchmarkDeleteInsert-4 1304244 1131 ns/op

BenchmarkDeleteInsertCloneOnceG-4 1834026 647.3 ns/op
BenchmarkDeleteInsertCloneOnce-4 1293346 932.3 ns/op

BenchmarkDeleteInsertCloneEachTimeG-4 545394 2878 ns/op
BenchmarkDeleteInsertCloneEachTime-4 353428 4173 ns/op

BenchmarkDeleteG-4 3223182 366.9 ns/op
BenchmarkDelete-4 1979107 600.4 ns/op

BenchmarkGetG-4 4265853 293.2 ns/op
BenchmarkGet-4 3091616 431.8 ns/op

BenchmarkGetCloneEachTimeG-4 1990131 693.3 ns/op
BenchmarkGetCloneEachTime-4 1705196 610.0 ns/op

BenchmarkAscendG-4 14222 83445 ns/op
BenchmarkAscend-4 12345 94486 ns/op

BenchmarkDescendG-4 14335 80779 ns/op
BenchmarkDescend-4 11686 98627 ns/op

BenchmarkAscendRangeG-4 8803 134915 ns/op
BenchmarkAscendRange-4 5586 209962 ns/op

BenchmarkDescendRangeG-4 6590 182179 ns/op
BenchmarkDescendRange-4 3591 323628 ns/op

BenchmarkAscendGreaterOrEqualG-4 12984 92049 ns/op
BenchmarkAscendGreaterOrEqual-4 9915 121264 ns/op

BenchmarkDescendLessOrEqualG-4 7876 152083 ns/op
BenchmarkDescendLessOrEqual-4 4945 231001 ns/op

BenchmarkDeleteAndRestoreG/CopyBigFreeList-4 100 10167381 ns/op 75390 B/op 15 allocs/op
BenchmarkDeleteAndRestore/CopyBigFreeList-4 73 15137467 ns/op 141924 B/op 19 allocs/op

BenchmarkDeleteAndRestoreG/Copy-4 104 12522643 ns/op 235632 B/op 1195 allocs/op
BenchmarkDeleteAndRestore/Copy-4 80 16853292 ns/op 433896 B/op 1151 allocs/op

BenchmarkDeleteAndRestoreG/ClearBigFreelist-4 207 5434805 ns/op 670 B/op 4 allocs/op
BenchmarkDeleteAndRestore/ClearBigFreelist-4 147 7954534 ns/op 980 B/op 6 allocs/op

BenchmarkDeleteAndRestoreG/Clear-4 198 6781077 ns/op 148424 B/op 1086 allocs/op
BenchmarkDeleteAndRestore/Clear-4 134 8639437 ns/op 268872 B/op 1041 allocs/op

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[gconnell]

Alternative fix to #41 that doesn't introduce a /v2/.

[AskAlexSharov]

is it backward compatible?

[gconnell]

Yes, it should be. I compiled and ran tests locally with Go 1.12 successfully.

[gconnell]

I've added a change to the GitHub actions to check all go versions from 1.11 through 1.18.

[gconnell]

Moving forward with this approach, but let me know if it causes any issues!