Resolve possible inference disruption by removing unneeded From impl

[dtolnay]

This impl breaks some code in rust-analyzer's ra_hir_def crate.

https://github.com/rust-analyzer/rust-analyzer/blob/1dba84019e0f3e7175f204624629a52013332e52/crates/ra_hir_def/src/path.rs#L273-L307

$ cargo check --manifest-path crates/ra_hir_def/Cargo.toml
    Checking ra_hir_def v0.1.0
    Finished dev [unoptimized] target(s) in 0.75s

$ cargo check --manifest-path crates/ra_hir_def/Cargo.toml --features log/kv_unstable
    Checking ra_hir_def v0.1.0
error[E0282]: type annotations needed for the closure `fn(&str) -> std::result::Result<(), _>`
   --> crates/ra_hir_def/src/path.rs:278:17
    |
278 |                 f.write_str("::")?;
    |                 ^^^^^^^^^^^^^^^^^^ cannot infer type
    |
help: give this closure an explicit return type without `_` placeholders
    |
276 |         let mut add_segment = |s| -> std::result::Result<(), _> {
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Closes #397.

[KodrAus]

Thanks @dtolnay!