const_fn

Accept const_fn attribute with no arguments and functions without const keyword.
This allows const_fn to be used as an optional dependency.

#[cfg_attr(feature = "...", const_fn::const_fn)]
pub fn func() {
    /* ... */
}

const_fn no longer fails to compile if unable to determine rustc version. Instead, it now displays a warning.

const_fn no longer relies on debug print format.

Improve error messages when failed to parse version information.

Fix compile failure with cargo installed by yum.

Add support for version-based code generation. The following conditions are available:

use const_fn::const_fn;

// function is `const` on specified version and later compiler (including beta and nightly)
#[const_fn("1.36")]
pub const fn version() {
    /* ... */
}

// function is `const` on nightly compiler (including dev build)
#[const_fn(nightly)]
pub const fn nightly() {
    /* ... */
}

// function is `const` if `cfg(...)` is true
#[const_fn(cfg(...))]
pub const fn cfg() {
    /* ... */
}

// function is `const` if `cfg(feature = "...")` is true
#[const_fn(feature = "...")]
pub const fn feature() {
    /* ... */
}

Improve compile time by removing proc-macro related dependencies (#18, #20).