Completed Exercise 41 - Maps

Simple word count. Had to import the strings package to split the
string.
Completed Exercise 44 - Fibonacci Closure

Keep all of the closure state in a slice. First element is number
returned. The other two elements hold the upcoming numbers for the next
function call.
Completed Exercise 48 - Complex Cube Roots

Had to strip out the short-circuit optimization from Exercise 24 because
there's no native way to compare complex numbers. 100 iterations seems
to work well.
Completed Exercise 56 - Errors

Conversion from float64 to string seem a little more complicated than
they should be, but whatever. It works.
Lesson 57 - Web Servers

This code could not run in the sandbox. The sandbox blocks opening
sockets and a web server by definition opens a server socket to listen
for web traffic. It needs to execute locally on the machine.
Completed Exercise 58 - HTTP Handlers

This was a cute simple web server with simple routing to types that
handle the responses. This is only good for simple servers. A real web
app needs a good templating engine to keep dev productivity up.