add rust-version attributes to cargo toml files

to better express/enforce our MSRV.

according to the cargo documentation ([1]) this was only introduced in
Rust version 1.56, but "older releases...will display a warning". as long
as it's truly just a warning and not an error, i'm okay with that.

[1]: https://doc.rust-lang.org/cargo/reference/manifest.html#the-rust-version-field

Author: jnqnfe

pulse-binding-mainloop-glib/Cargo.toml

@@ -10,6 +10,7 @@ keywords = ["binding", "pulseaudio", "glib", "audio"]
 homepage = "https://github.com/jnqnfe/pulse-binding-rust"
 repository = "https://github.com/jnqnfe/pulse-binding-rust"
 exclude = ["README.md"]
+rust-version = "1.56"
 
 [dependencies]
 libpulse-binding           = { path = "../pulse-binding",           version = "2.25", default-features = false }

pulse-binding-simple/Cargo.toml

@@ -10,6 +10,7 @@ keywords = ["binding", "pulseaudio", "audio"]
 homepage = "https://github.com/jnqnfe/pulse-binding-rust"
 repository = "https://github.com/jnqnfe/pulse-binding-rust"
 exclude = ["README.md"]
+rust-version = "1.46"
 
 [dependencies]
 libpulse-binding    = { path = "../pulse-binding",    version = "2.24", default-features = false }

pulse-binding/Cargo.toml

@@ -10,6 +10,7 @@ keywords = ["binding", "pulseaudio", "audio"]
 homepage = "https://github.com/jnqnfe/pulse-binding-rust"
 repository = "https://github.com/jnqnfe/pulse-binding-rust"
 exclude = ["README.md"]
+rust-version = "1.46"
 
 [dependencies]
 libc = "0.2"

pulse-sys-mainloop-glib/Cargo.toml

@@ -12,6 +12,7 @@ repository = "https://github.com/jnqnfe/pulse-binding-rust"
 exclude = ["README.md"]
 links = "pulse-mainloop-glib"
 build = "build.rs"
+rust-version = "1.56"
 
 [dependencies]
 libpulse-sys = { path = "../pulse-sys", version = "1.19", default-features = false }

pulse-sys-simple/Cargo.toml

@@ -12,6 +12,7 @@ repository = "https://github.com/jnqnfe/pulse-binding-rust"
 exclude = ["README.md"]
 links = "pulse-simple"
 build = "build.rs"
+rust-version = "1.46"
 
 [dependencies]
 libpulse-sys = { path = "../pulse-sys", version = "1.19", default-features = false }

pulse-sys/Cargo.toml

@@ -12,6 +12,7 @@ repository = "https://github.com/jnqnfe/pulse-binding-rust"
 exclude = ["README.md"]
 links = "pulse"
 build = "build.rs"
+rust-version = "1.46"
 
 [dependencies]
 libc = "0.2"