[ruby/strscan] Micro optimize encoding checks

(https://github.com/ruby/strscan/pull/117)

Profiling shows a lot of time spent in various encoding check functions.
I'm working on optimizing them on the Ruby side, but if we assume most
strings are one of the simple 3 encodings, we can skip a lot of
overhead.

```ruby
require 'strscan'
require 'benchmark/ips'

source = 10_000.times.map { rand(9999999).to_s }.join(",").force_encoding(Encoding::UTF_8).freeze

def scan_to_i(source)
  scanner = StringScanner.new(source)
  while number = scanner.scan(/\d+/)
    number.to_i
    scanner.skip(",")
  end
end

def scan_integer(source)
  scanner = StringScanner.new(source)
  while scanner.scan_integer
    scanner.skip(",")
  end
end

Benchmark.ips do |x|
  x.report("scan.to_i") { scan_to_i(source) }
  x.report("scan_integer") { scan_integer(source) }
  x.compare!
end
```

Before:

```
ruby 3.3.4 (2024-07-09 revision https://github.com/ruby/strscan/commit/be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
           scan.to_i    93.000 i/100ms
        scan_integer   232.000 i/100ms
Calculating -------------------------------------
           scan.to_i    933.191 (± 0.2%) i/s    (1.07 ms/i) -      4.743k in   5.082597s
        scan_integer      2.326k (± 0.8%) i/s  (429.99 μs/i) -     11.832k in   5.087974s

Comparison:
        scan_integer:     2325.6 i/s
           scan.to_i:      933.2 i/s - 2.49x  slower
```

After:

```
ruby 3.3.4 (2024-07-09 revision https://github.com/ruby/strscan/commit/be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
           scan.to_i    96.000 i/100ms
        scan_integer   274.000 i/100ms
Calculating -------------------------------------
           scan.to_i    969.489 (± 0.2%) i/s    (1.03 ms/i) -      4.896k in   5.050114s
        scan_integer      2.756k (± 0.1%) i/s  (362.88 μs/i) -     13.974k in   5.070837s

Comparison:
        scan_integer:     2755.8 i/s
           scan.to_i:      969.5 i/s - 2.84x  slower
```

https://github.com/ruby/strscan/commit/c02b1ce684

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