Improve plural operand handling

[echeran]

In case we're not already doing it, we should make sure that when parsing a string representing a number and/or performing PluralRules::select():

• we parse both compact notation (1.2c3) and scientific notation (1.2e)
• distinguish the c operand from the e operand, to future-proof when those behave differently
• make sure we compute plural operands correctly for numbers with exponents (the examples list from the latest spec makes for good test cases)

[sffc]

@eggrobin Could you take a look at the bullet points in this issue and determine what needs to be done?

[sffc]

@eggrobin Please take a look at this; thanks!