Replace boost/algorithm with standalone string utils

[andrjohns]

Submission Checklist

• Run unit tests: ./runTests.py src/test/unit
• Run cpplint: make cpplint
• Declare copyright holder and open-source license: see below

Summary

Created a new stan/io/string_utils.hpp header with the minimum needed string utility functions for dropping our dependency on boost/algorithm

Intended Effect

Reduce Boost dependency footprint

How to Verify

Existing tests/models all still pass

Side Effects

N/A

Documentation

N/A

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Andrew Johnson

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)