msrv 1.57.0 may need to be bumped

[WesleyRosenblum]

Building bolero on Rust 1.57.0 doesn't seem possible anymore, and even Rust 1.63.0 has several issues with transitive dependencies:

package winnow v0.5.14 cannot be built because it requires rustc 1.64.0 or newer, while the currently active rustc version is 1.63.0

package addr2line v0.21.0 cannot be built because it requires rustc 1.65 or newer, while the currently active rustc version is 1.63.0

package hashbrown v0.14.0 cannot be built because it requires rustc 1.64.0 or newer, while the currently active rustc version is 1.63.0

package toml_datetime v0.6.3 cannot be built because it requires rustc 1.64.0 or newer, while the currently active rustc version is 1.63.0

I can get it to build on Rust 1.63.0 by pinning the following dependencies:

backtrace = { version = "=0.3.68" } (in bolero-engine)
proc-macro-crate = "=1.3.0" (in bolero-generator-derive)