Add comments for things that surprised me

As a new user, there were things I was skeptical about and after digging
in, these were my conclusions.

Compared to the simple and obvious lua wiki solutions, batteries' string
functions are slightly faster. GC is the same.

Test
    local str = "hello world"
    local fn = function()
        local x = 0
        if stringx.ends_with(str, "h") then
            x = x + 1
        end
        if stringx.ends_with(str, "helll") then
            x = x + 1
        end
        if stringx.ends_with(str, "helicopter") then
            x = x + 1
        end
    end
    local pretty = require "inspect"
    print("stringx =", pretty({
                time_taken = {measure.time_taken(fn, 10000)},
                memory_taken = {measure.memory_taken(fn, 10000)}
        }))
    local function starts_with(str, prefix)
        return str:find(prefix, 1, true) == 1
    end
    local function ends_with(str, ending)
        return ending == "" or str:sub(-#ending) == ending
    end
    local fn = function()
        local x = 0
        if ends_with(str, "h") then
            x = x + 1
        end
        if ends_with(str, "helll") then
            x = x + 1
        end
        if ends_with(str, "helicopter") then
            x = x + 1
        end
    end
    print("find =", pretty({
                time_taken = {measure.time_taken(fn, 10000)},
                memory_taken = {measure.memory_taken(fn, 10000)}
        }))

starts_with
===========

stringx =       {
  memory_taken = { 0, 0, 0 },
  time_taken = { 1.5098012518138e-007, 9.988434612751e-008, 2.1699932403862e-005 }
}
find =  {
  memory_taken = { 0, 0, 0 },
  time_taken = { 2.7349997544661e-007, 1.9988510757685e-007, 9.1999536380172e-006 }
}

ends_with
=========

stringx =       {
  memory_taken = { 0, 0, 0 },
  time_taken = { 9.0479978825897e-008, 0, 2.5199959054589e-005 }
}
find =  {
  memory_taken = { 0, 0, 0 },
  time_taken = { 2.1833006758243e-007, 1.9988510757685e-007, 6.1000464484096e-006 }
}

set.lua

@@ -1,5 +1,8 @@
 --[[
 	set type with appropriate operations
+
+	NOTE: This is actually a unique list (ordered set). So it's more than just
+	a table with keys for values.
 ]]
 
 local path = (...):gsub("set", "")

stringx.lua

@@ -249,6 +249,7 @@ end
 
 --check if a given string starts with another
 --(without garbage)
+--Using loops is actually faster than string.find!
 function stringx.starts_with(s, prefix)
 	for i = 1, #prefix do
 		if s:byte(i) ~= prefix:byte(i) then