Use memoization for unbracketed GoldenRatio

[sebasv]

Concerns calls to GoldenRatio::minimize:

(paraphrased)
If the function values generated in GoldenRatio::explore are stored in a key-value map, and then instead of the precise golden ratio just a "close enough" value from the map is used, the number of evaluations in GoldenRatio::minimize_bracket is potentially lowered at the cost of space.

Originally posted by @to266 in #6