Optimize Util.valid_variable_name? method

[fatkodima]

Production profiling of our application revealed that a single Stripe::Util.valid_variable_name? method was using a few % of its runtime. This PR optimizes that method.

[cla-assistant]

All committers have signed the CLA.

[prathmesh-stripe]

fatkodima Please add more details to the PR. Some metrics, screenshots and a way to reproduce this would be helpful to evaluate this PR.

[fatkodima]

Production profiles:

CPU

       538   (1.7%)         538   (1.7%)     Regexp#match?
     26545  (83.0%)         419   (1.3%)     Array#each
       405   (1.3%)         405   (1.3%)     String#[]
       389   (1.2%)         389   (1.2%)     String#=~  <========== this line
       345   (1.1%)         345   (1.1%)     String#sub

Allocations

      3250   (3.6%)        3250   (3.6%)     String#split
      3199   (3.6%)        3199   (3.6%)     String#=~  <===============
      2315   (2.6%)        2269   (2.5%)     PG::Result#values
      7629   (8.5%)        1957   (2.2%)     Thread.each_caller_location
      1922   (2.2%)        1922   (2.2%)     Thread::Backtrace::Location#to_s
      1860   (2.1%)        1860   (2.1%)     PG::Result#fields
      1728   (1.9%)        1723   (1.9%)     ActiveRecord::ConnectionAdapters::PostgreSQLAdapter#get_oid_type
      4509   (5.1%)        1597   (1.8%)     Kernel#dup
      1400   (1.6%)        1400   (1.6%)     String#byteslice
      1096   (1.2%)        1072   (1.2%)     ActiveRecord::Associations#init_internals
       846   (0.9%)         846   (0.9%)     String#chars <===============
       838   (0.9%)         838   (0.9%)     Integer#to_s

I don't have a benchmark, but it is easy to see that it allocates a chars array (.chars.) each time and creates regexp captures (because of =~), even though these are not needed.

[prathmesh-stripe]

Thanks for submitting this PR. Tested the new regex against:

Variable	Valid/Invalid	Comments
foo	✅	Starts with letter, contains letters
_private	✅	Starts with underscore
foo123	✅	Letters + digits
a	✅	Single letter
123foo	❌	Starts with digit
my-foo	❌	Contains hyphen
my foo	❌	Contains space
foo!	❌	Contains special character
``	❌	Empty string