Adjust practice exercise metadata (difficulty, practices, prerequisites)

[IsaacG]

This makes some inroads on #1396

After these changes, Concepts Practiced:

» ./concepts.py
=> Present in concepts, missing in exercises: ['constants', 'error-wrapping', 'multiple-return-values', 'variables']

Number of practices for a concept (ideally 3-8):
00: comments, conditionals-switch, functions, pointers, range-iteration, regular-expressions, stringers, strings, strings-package, structs, type-assertion, type-conversion, type-definitions, zero-values
01: comparison, conditionals-if, packages
02: booleans, first-class-functions, floating-point-numbers, interfaces, methods, numbers, randomness, runes, time
03: maps, string-formatting, variadic-functions
04: arithmetic-operators, basics, errors, slices
05: for-loops

4 practices listed in collatz-conjecture