Improve performance by using bytesize when comparing if the string changed

[davekaro]

length slows down as the string grows (appears to be O(n)). bytesize stays the same O(1). Even on very small strings like "<h1>", bytesize is much faster than length. By swapping over to bytesize we can see an overall improvement in Phlex bench.rb:

Before (using length):

ruby 3.3.0 (2023-12-25 revision 5124f9ac75) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                Page    13.160k i/100ms
Calculating -------------------------------------
                Page    133.259k (± 0.9%) i/s -    671.160k in   5.036897s

After (using bytesize):

ruby 3.3.0 (2023-12-25 revision 5124f9ac75) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                Page    15.000k i/100ms
Calculating -------------------------------------
                Page    154.814k (± 1.2%) i/s -    780.000k in   5.039022s

If we further tune bench.rb as I've done in this PR to more accurately represent what might be a typical "string length" when using Phlex (think larger components or full page views), we see an even more dramatic improvement:

Before (using length on larger string):

ruby 3.3.0 (2023-12-25 revision 5124f9ac75) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                Page    73.000 i/100ms
Calculating -------------------------------------
                Page    734.131 (± 0.4%) i/s -      3.723k in   5.071408s

After (using bytesize on larger string):

ruby 3.3.0 (2023-12-25 revision 5124f9ac75) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                Page   520.000 i/100ms
Calculating -------------------------------------
                Page      5.243k (± 0.6%) i/s -     26.520k in   5.058093s

Comes to about a 7x speed increase. Always faster, with potential for huge gains. I had a page go from 15 seconds to 300ms with this change (50x speedup). This was a rare page of over 3 megabytes of HTML all generated by Phlex, but still pretty fun.