move some boilerplate code having to do with overflow math safety
to functions.

Again the timing difference is not much:

Before:

./build size: 1.3 MB
hello.zig size: 308 KB
full test:  1m33.588s
debug test: 20.303s
hello.zig timing:
                Name       Start         End    Duration     Percent
          Initialize      0.0000      0.0000      0.0000      0.0002
   Semantic Analysis      0.0000      0.0425      0.0425      0.2202
     Code Generation      0.0425      0.0675      0.0250      0.1293
    LLVM Emit Object      0.0675      0.1789      0.1114      0.5773
  Build Dependencies      0.1789      0.1913      0.0124      0.0640
           LLVM Link      0.1913      0.1931      0.0018      0.0091
         Generate .h      0.1931      0.1931      0.0000      0.0000
               Total      0.0000      0.1931      0.1931      1.0000

After:

./build size: 1.3 MB
hello.zig size: 301 KB
full test: 1m31.253s
debug test: 19.607s
hello.zig timing:
                Name       Start         End    Duration     Percent
          Initialize      0.0000      0.0000      0.0000      0.0002
   Semantic Analysis      0.0000      0.0431      0.0431      0.2262
     Code Generation      0.0431      0.0660      0.0229      0.1201
    LLVM Emit Object      0.0660      0.1765      0.1105      0.5795
  Build Dependencies      0.1765      0.1890      0.0125      0.0655
           LLVM Link      0.1890      0.1906      0.0016      0.0086
         Generate .h      0.1906      0.1906      0.0000      0.0000
               Total      0.0000      0.1906      0.1906      1.0000