Switch radix to an enum for easier formatting (#1130)

Note the use of an enum allows for the operator to be defined for formatting. This reduces the custom formatting required, which is a simplification carrying over to error handling changes I'm working on too.

Also, I kind of feel the code's now clearer about what's supported; I'm dropping CHECKs that seemed redundant with the enum.

toolchain/lexer/tokenized_buffer.cpp

@@ -252,7 +252,7 @@ class TokenizedBuffer::Lexer {
           buffer_.literal_int_storage_.push_back(std::move(value.mantissa));
           buffer_.literal_int_storage_.push_back(std::move(value.exponent));
           CHECK(buffer_.GetRealLiteral(token).IsDecimal() ==
-                (value.radix == 10));
+                (value.radix == LexedNumericLiteral::Radix::Decimal));
           return token;
         },
         [&](LexedNumericLiteral::UnrecoverableError) {