Revamp compiler messages to be more friendly

[paulcsmith]

Part of #7553 and https://forum.crystal-lang.org/t/rfc-revamp-how-compile-time-errors-are-shown/548/4

Type must be String, got Int32

Instead: 'my_method' should return a String, but it returned an Int32'

Undefined constant

I couldn't find the type or constant 'Foo'

Please suggest more if you run into them :D