Implement #48in24 exercises

[ErikSchierboom]

The following #48in24 exercises haven't yet been implemented (in order of week they're featured in):

• [x] parallel-letter-frequency
• [x] minesweeper
• [x] dnd-character
• [x] yacht
• [x] secret-handshake
• [x] robot-simulator
• [x] meetup
• [x] phone-number
• [x] two-bucket
• [x] grade-school
• [x] bank-account
• [x] food-chain

[ErikSchierboom]

Note: circular-buffer is foregone as you can't really easily create a fixed-size, mutable backing array.