StringStorage tried to adopt the visitor pattern, but it regressed
benchmarks too much. We'd like to come up with a mutation story
anyways, and StringStorage was sort of cheating that by being a class.
Restores perf-regressions of Join etc.
Use the visitor pattern in most of the opaque-by-hand call
sites. Inspecting the compiler output does not show excessive and
unanticipated ARC, but there may need to be further tweaks.
One downside of the visitor pattern as written is that there's extra
shuffling around of registers for the closure CC. Hopefully this will
also be fixed soon.
Stop inlining _asOpaque into user code. Inlining it bloats user code
as there's a bit-test-and-branch to a block containing the _asOpaque
call, followed up some operations to e.g. manipulate the range or
re-align the calling convention, etc., followed by a final branch to
opaque stdlib code.
Instead, branch directly into opaque stdlib code. In theory, this
means that supporting all opaque patterns can be done with minimal
bloat. On ARM, this is a single tbnz instruction.
Include the initial implementation of _StringGuts, a 2-word
replacement for _LegacyStringCore. 64-bit Darwin supported, 32-bit and
Linux support in subsequent commits.
