codeframe in assertion failures

[aaronabramov]

we have line! and column! macros. we can technically take out a pice of code with some context around it where assertion failed and add it to the error message
e.g.

Assertion Failure!

43 | fn do something() {
44 |   let stuff = 1
45 |   assert_equal!(stuff, 2);
46 |  }

[TheWebDevel]

Hi @aaronabramov,

I made a prototype of code-frame: https://github.com/TheWebDevel/codeframe-prototype.