Load new operators at runtime

[nick-paul]

This is a continuation & modification of #111

I changed the loading process to an operator instead of loading from the libs folder on startup. This was a suggested option in the original PR. To load additional operators, you can use the :(library.load) named operator.

I went with :(library.load) instead of auto loading from libs because I prefer to allow Aya code to have more control over how and when to load libraries at runtime. This will be useful for adding a package manager or improving importlib.

Example

For an example, check out the example library based off the sample library provided in the original PR here:

https://github.com/aya-lang/aya-library-example

aya> "path/to/aya-library-example-1.0-SNAPSHOT.jar" :(library.load)
[ ":(example.instruction)" ":(example.instruction2)" ]
aya> :(example.instruction)
"hello, world"
aya> :(example.instruction2)
"foo bar"

You can also use the import statement to load the jar. This will not return the list of loaded operators

aya> import "path/to/aya-library-example-1.0-SNAPSHOT.jar"
aya> :(example.instruction)
"hello, world"