Add f32_float feature.

Cargo.toml

@@ -32,6 +32,7 @@ unchecked = []      # unchecked arithmetic
 sync = []           # restrict to only types that implement Send + Sync
 no_optimize = []    # no script optimizer
 no_float = []       # no floating-point
+f32_float = []      # set FLOAT=f32
 only_i32 = []       # set INT=i32 (useful for 32-bit systems)
 only_i64 = []       # set INT=i64 (default) and disable support for all other integer types
 no_index = []       # no arrays and indexing