Add Conv2d

[coreylowman]

Closes #1

• Add tests for conv2d & batch_conv2d
• Add tests for Conv2D layer
• Add example file
• Clean up implementation

[coreylowman]

Adding a const CFG: ConvConfig could be really helpful to reduce const generic parameters here. Will require an additional feature to enable usage of structs in const

[M1ngXU]

this could be really helpful, especially if the amount of filters can be set independently, so there could be a default convconfig for a kernel size of 3x3 with 0 padding etc. And conv2d is nightly anyway, so another feature wouldn't hurt much

[M1ngXU]

a default option like this could use typealiasing though and would not require extra-features