Support non-nightly version of Rust (beta or stable)

[iceiix]

Steven requires nightly Rust since it uses experimental features. To improve maintainability, would be nice to adapt it to work on either the beta or preferably stable version of Rust.

[iceiix]

It uses one experimental feature, in src/main.rs: #![feature(const_fn)]. This feature is coming soon to stable Rust: https://www.reddit.com/r/rust/comments/9msqfn/const_fn_soon_on_stable_rust/, however, removing the feature flag (and testing on nightly-2018-10-24), it is not the full const_fn, because: https://github.com/rust-lang/rust/issues/24111#issuecomment-414385646:

Another thing that came up on Discord are FP operations: We cannot currently guarantee that they will match real hardware. CTFE will follow IEEE exactly, but LLVM/hardware might not.

Instead, a subset of const_fn, called min_const_fn, was enabled. So removing the feature flag:

--- a/src/main.rs
+++ b/src/main.rs
@@ -13,7 +13,6 @@
 // limitations under the License.
 
 #![recursion_limit="300"]
-#![feature(const_fn)]
 
 extern crate sdl2;
 extern crate zip;

causes this compilation failure:

error: trait bounds other than `Sized` on const fn parameters are unstable                                                                                                                                         
  --> src/types/metadata.rs:30:7                                                                                                                                                                                   
   |                                                                                                                                                                                                               
30 | impl <T: MetaValue> MetadataKey<T> {                                                                                                                                                                          
   |       ^                                                                                                                                                                                                       
                                                                                                                                                                                                                   
error: only int, `bool` and `char` operations are stable in const fn                                                                                                                                               
  --> src/world/biome.rs:16:23                                                                                                                                                                                     
   |                                                                                                                                                                                                               
16 |             moisture: m*t,                                                                                                                                                                                    
   |                       ^^^                                                                                                                                                                                     
                                                                                                                                                                                                                   
error: aborting due to 2 previous errors                     

src/world/biome.rs uses const fn to define constant biomes, with temperature and moisture, both floating-point (f64). src/types/metadata.rs uses it to construct MetadataKey from i32 index.

[iceiix]

Rust 2018 is coming up, timely to target it.

Would also simplify the installation steps: https://github.com/Thinkofname/steven/wiki/Compiling-and-or-running documents nightly is required.