[v2] Replace error type with ones more like std

[CAD97]

A start towards #49, though I haven't audited panicking, just migrated the existing errors.

Notably, because the error type no longer holds String, the crate no longer has a hard requirement on alloc.

[paupino]

Thank you for this! I'll need to start a v2 branch to start API breaking changes. There are a few things I'd ultimately like to address/clean up for v2 while at the same time still be able to bug fix any smaller issues on the v1 branch.

For the meantime, I'll leave this open and once I create a v2 branch we'll see if we can retarget it and perform a full review. There are a couple of v1 changes I'd like to get in before creating the branch (just to make it easier).