Refactor handling of width and precision formatting options

These formatting options may be either a size specified in the format
string itself or a nested replacement field whose value determines the
size at runtime. Rather than tracking each potential option as its own
std::optional<std::size_t>, introduce a small structure to encapsulate
the option's value and type (a static size or format parameter).

Further, only string and floating point values are affected by the
precision option. Defer handling precision to the type-specific
overloads of BasicStringFormatter::format_value_for_type.

Author: trflynn89