Clippy complains about tame_index::Error being very large in size

[Shnatsel]

Describe the bug
Clippy complains about tame_index::Error being very large:

 error: the `Err`-variant returned from this function is very large
Error:   --> rustsec/src/cached_index.rs:77:10
   |
77 |     ) -> Result<Self, tame_index::Error> {
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the `Err`-variant is at least 128 bytes
   |
   = help: try reducing the size of `tame_index::Error`, for example by boxing large elements or replacing it with `Box<tame_index::Error>`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#result_large_err

For structs of over 128 bytes passed by value LLVM invokes memcpy(), so propagating the error up the call stack with ? over multiple functions involves multiple memcpy() calls. This is rather slow, although it is unlikely to be a problem for tame-index in practice since most of the time is spent waiting on the network anyway.

To Reproduce
cargo clippy

Device:
Clippy 1.67.0 on Linux x86_64. Doesn't happen in later Clippy versions, which is peculiar.

[Jake-Shadle]

That error is incorrect, the largest size is 120 bytes, so maybe that old version of clippy was calculating it incorrectly.

print-type-size type: `error::Error`: 120 bytes, alignment: 8 bytes
print-type-size     variant `Git`: 120 bytes
print-type-size         field `.0`: 120 bytes
print-type-size     variant `Toml`: 96 bytes
print-type-size         field `.0`: 96 bytes
print-type-size     variant `Local`: 56 bytes
print-type-size         field `.0`: 56 bytes
print-type-size     variant `IoPath`: 32 bytes
print-type-size         field `.0`: 8 bytes
print-type-size         field `.1`: 24 bytes
print-type-size     variant `InvalidUrl`: 32 bytes
print-type-size         field `.0`: 32 bytes
print-type-size     variant `NonUtf8Path`: 24 bytes
print-type-size         field `.0`: 24 bytes
print-type-size     variant `InvalidKrateName`: 24 bytes
print-type-size         field `.0`: 24 bytes
print-type-size     variant `Http`: 24 bytes
print-type-size         field `.0`: 24 bytes
print-type-size     variant `Io`: 8 bytes
print-type-size         field `.0`: 8 bytes
print-type-size     variant `Json`: 8 bytes
print-type-size         field `.0`: 8 bytes
print-type-size     variant `Semver`: 8 bytes
print-type-size         field `.0`: 8 bytes
print-type-size     variant `Cache`: 1 bytes
print-type-size         field `.0`: 1 bytes
print-type-size     variant `NoCrateVersions`: 0 bytes

But Git is still larger than most of the others by a wide margin. I'll slim it down, again.