This method is the 4th biggest source of allocation during our application
boot (~300k allocations).

The goal of this PR was mostly to reduce allocations, but it also translate
to a 5-17% speedup.

The main change is leveraging `capitalize!` rather than `capitalize`.

The second important change is to merge the two `gsub!` together.

```ruby
require 'benchmark/ips'
require 'active_support/all'

module ActiveSupport
  module Inflector
    def camelize2(term, uppercase_first_letter = true)
      string = term.to_s
      if uppercase_first_letter
        string = string.sub(/^[a-z\d]*/) { |match| inflections.acronyms[match] || match.capitalize! || match }
      else
        string = string.sub(inflections.acronyms_camelize_regex) { |match| match.downcase! || match }
      end
      string.gsub!(/(?:_|(\/))([a-z\d]*)/i) do
        if $1
          "::#{inflections.acronyms[$2] || $2.capitalize! || $2}"
        else
          inflections.acronyms[$2] || $2.capitalize! || $2
        end
      end
      string
    end
  end
end

%w(foo foo_bar foo/bar_baz).each do |str|
  puts "== Comparing with #{str.inspect} (#{RUBY_VERSION}) =="
  Benchmark.ips do |x|
    x.report('camelize') { ActiveSupport::Inflector.camelize(str) }
    x.report('camelize2') { ActiveSupport::Inflector.camelize2(str) }
    x.compare!
  end
  puts
end
```

Results
```
== Comparing with "foo" (2.7.2) ==
Warming up --------------------------------------
            camelize    53.646k i/100ms
           camelize2    56.797k i/100ms
Calculating -------------------------------------
            camelize    532.012k (± 1.3%) i/s -      2.682M in   5.042620s
           camelize2    567.317k (± 1.3%) i/s -      2.840M in   5.006633s

Comparison:
           camelize2:   567317.4 i/s
            camelize:   532012.3 i/s - 1.07x  (± 0.00) slower

== Comparing with "foo_bar" (2.7.2) ==
Warming up --------------------------------------
            camelize    24.010k i/100ms
           camelize2    25.398k i/100ms
Calculating -------------------------------------
            camelize    239.036k (± 1.1%) i/s -      1.200M in   5.022811s
           camelize2    254.229k (± 1.3%) i/s -      1.295M in   5.095855s

Comparison:
           camelize2:   254228.8 i/s
            camelize:   239036.4 i/s - 1.06x  (± 0.00) slower

== Comparing with "foo/bar_baz" (2.7.2) ==
Warming up --------------------------------------
            camelize    17.116k i/100ms
           camelize2    19.628k i/100ms
Calculating -------------------------------------
            camelize    169.210k (± 1.0%) i/s -    855.800k in   5.058138s
           camelize2    199.385k (± 2.1%) i/s -      1.001M in   5.022935s

Comparison:
           camelize2:   199384.7 i/s
            camelize:   169210.0 i/s - 1.18x  (± 0.00) slower
```