[keygen] Remove deprecated functions from the grind command

[samkim-crypto]

Problem

The solana-keygen cli still has some deprecated functions from clap-v2 for the grind command logic.

Summary of Changes

Removed deprecated functions from the grind command.

I think most commits should be pretty straightforward except for f4aee92, which replaces grind validator functions with a united grind parser. It would be nice to implement a parser that returns a GrindMatch type directly so that we can run matches.get_one::<GrindMatch>(...), but the parsing logic depends on the ignore_case argument, which makes this hard to do. I ended up just defining String parser that executes the existing validation logic for grind arguments.

This is a follow-up to #438.

Fixes #

[CriesofCarrots]

Just nits. It's too bad about the GrindMatch parser, but I can't think of a better approach, given the clap api.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.8%. Comparing base (212cbdb) to head (065267f).
Report is 29 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #490   +/-   ##
=======================================
  Coverage    81.8%    81.8%           
=======================================
  Files         842      842           
  Lines      228436   228436           
=======================================
+ Hits       186996   187008   +12     
+ Misses      41440    41428   -12