Feat: stdlib: adds std/algorithm.sortedBy

[nervecenter]

Adds sortedBy, a convenience proc around sorted. The closure comparable should produce a single comparable value for each element, which will then be used to sort with system.cmp by order. Intended to be a generically typed alternative to sortedByIt to avoid the template. Here's my runnableExamples:

import std/math
import std/sugar
import std/sequtils

type Country = tuple[name: string, gdp: float, area: float]

let countries: seq[Country] = @[
  (name: "USA",     gdp: 30.617, area: 9.148),
  (name: "China",   gdp: 19.399, area: 9.326),
  (name: "Germany", gdp: 5.015,  area: 0.349),
  (name: "Japan",   gdp: 4.281,  area: 0.364),
  (name: "India",   gdp: 4.028,  area: 2.973)
]

assert countries.sortedBy(c => c.gdp, Descending).map(c => c.name) == @["USA",
  "China", "Germany", "Japan", "India"]
assert countries.sortedBy(c => c.area, Descending).map(c => c.name) == @["China",
  "USA", "India", "Japan", "Germany"]
assert countries.sortedBy(c => c.area.pow(2.0), Descending).map(c => c.name) == @["China",
  "USA", "India", "Japan", "Germany"]
assert countries.sortedBy(c => c.name).map(c => c.name) == @["China",
  "Germany", "India", "Japan", "USA"]
assert countries.sortedBy(c => c.name[^1]).map(c => c.name) == @["USA",
  "China", "India", "Japan", "Germany"]