Serious performance regression in Grow method of OutputBuffer

[eitanhs]

Recently, in order to mitigate CVE-2020-1469, among other changes, line 224 of OutputBuffer class
changed from length += length >> 1; to length = checked(length + length >> 1);
This actually doesn't change the value in length variable, because the + operator precedes the bit-shift operator,
So basically we multiply and divide == no change.
This causes serious performance regression as the underlying buffer grows by single value at a time (instead of growing by half of current size)...

I'll open a PR with a fix suggestion soon.

[chwarr]

Nice find! Fix merged into master.