Preparing to custom build the string ctor at compile time
Remove redundant assertion
Removing the exponentiation of ten improves the small case

Here are some average timings after this commit

Average initialization duration of Pi to 10 digits using std.BigInt = 878.02 ns
Average initialization duration of Pi to 10 digits using phi.Integer = 1394.12 ns
Average initialization duration of Pi to 50 digits using std.BigInt = 1177.84 ns
Average initialization duration of Pi to 50 digits using phi.Integer = 4473.6 ns
Average initialization duration of Pi to 100 digits using std.BigInt = 1644.74 ns
Average initialization duration of Pi to 100 digits using phi.Integer = 10299.5 ns
Average initialization duration of Pi to 1000 digits using std.BigInt = 24691.6 ns
Average initialization duration of Pi to 1000 digits using phi.Integer = 484568 ns

Here are the average timings from the previous run (see issue #37)

Average initialization duration of Pi to 10 digits using std.BigInt = 807.71 ns
Average initialization duration of Pi to 10 digits using phi.Integer = 1532.41 ns
Average initialization duration of Pi to 50 digits using std.BigInt = 1186.06 ns
Average initialization duration of Pi to 50 digits using phi.Integer = 4557.75 ns
Average initialization duration of Pi to 100 digits using std.BigInt = 1649.94 ns
Average initialization duration of Pi to 100 digits using phi.Integer = 10455.1 ns
Average initialization duration of Pi to 1000 digits using std.BigInt = 24782.2 ns
Average initialization duration of Pi to 1000 digits using phi.Integer = 487600 ns
Keep the style uniform
More of the same
Removed an unnecessary modulus division

Average initialization duration of Pi to 10 digits using std.BigInt = 975.64 ns
Average initialization duration of Pi to 10 digits using phi.Integer = 1351.57 ns
Average initialization duration of Pi to 50 digits using std.BigInt = 1196.82 ns
Average initialization duration of Pi to 50 digits using phi.Integer = 4326.21 ns
Average initialization duration of Pi to 100 digits using std.BigInt = 1705.23 ns
Average initialization duration of Pi to 100 digits using phi.Integer = 9573.1 ns
Average initialization duration of Pi to 1000 digits using std.BigInt = 24341.3 ns
Average initialization duration of Pi to 1000 digits using phi.Integer = 423041 ns