prepare deprecation of simple and alternative

[jverzani]

This PR does:

• streamlines fzero
• separates out code and tests for alternative_interfaces and simple. (some goes into find_zeros)
• prepares deprecation warnings for functions in alternative_interfaces and simple (but doesn't uncomment them yet)
• add modab code to simple, as it was contributed and might be of independent interest

The goal is to deprecate the two files, both could be elsewhere, especially alternative interfaces which has had comments about it being legacy for years now.

[codecov]

Codecov Report

❌ Patch coverage is 62.65060% with 62 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.96%. Comparing base (a45a34f) to head (aaac10e).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/deprecated/simple.jl	14.49%	59 Missing ⚠️
src/find_zeros.jl	96.55%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #507      +/-   ##
==========================================
- Coverage   86.93%   82.96%   -3.98%     
==========================================
  Files          41       41              
  Lines        2565     2607      +42     
==========================================
- Hits         2230     2163      -67     
- Misses        335      444     +109     

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