Remove trait bound from Error type

[CAD97]

Fixes #693. Let's see what's wrong with the simple fix.

[tomtau]

lgtm, likely this is fine, but I'd want to try if no_std is fine; i'll try to run (or anyone can try) rustup run nightly -- cargo build -j1 -Z build-std=core,alloc --target x86_64-unknown-linux-gnu --no-default-features on pest before merging it

[CAD97]

CAD97-patch-1:pest〉cargo clean
CAD97-patch-1:pest〉cargo bootstrap
   Compiling proc-macro2 v1.0.41
   Compiling unicode-ident v1.0.2
   Compiling quote v1.0.20
   Compiling syn v1.0.98
   Compiling ucd-trie v0.1.4
   Compiling maplit v1.0.2
   Compiling pest v2.1.3
   Compiling pest_meta v2.1.3
   Compiling pest_generator v2.1.3
   Compiling pest_bootstrap v0.0.0 (D:\git\pest-parser\pest\bootstrap)
    Finished dev [unoptimized + debuginfo] target(s) in 3.92s
     Running `D:\.rust\target\debug\pest_bootstrap.exe`
CAD97-patch-1:pest〉rustup run nightly -- cargo build -j1 -Z build-std=core,alloc --target x86_64-pc-windows-msvc -p pest --no-default-features
   Compiling compiler_builtins v0.1.79
   Compiling core v0.0.0 (D:\.rust\rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core)
   Compiling rustc-std-workspace-core v1.99.0 (D:\.rust\rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-core)
   Compiling alloc v0.0.0 (D:\.rust\rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\alloc)
   Compiling ucd-trie v0.1.4
   Compiling pest v2.2.1 (D:\git\pest-parser\pest\pest)
    Finished dev [unoptimized + debuginfo] target(s) in 14.32s