(third) Implement formatToParts

[stasm]

This is a prototype implementation of the formatToParts iterator discussed in #41. The approach taken is the one from #41 (comment) by @eemeli with one modification:

• the iterator yields a flat sequence of parts,
• a part can either be a FormattedPart {type: string, value: string} or an OpaquePart,
• FormattedPart doesn't have the source field carrying the original raw value; it was easier to do it that way but also it's consistent with existing formatToParts specifications,
• patterns coming from message references are flattened into the final sequence.

Examples:

let payloadSize = 1.23;
"Transferred {NUMBER $payloadSize STYLE unit UNIT megabyte}."

{ type: 'literal', value: 'Transferred ' },
{ type: 'integer', value: '1' },
{ type: 'decimal', value: '.' },
{ type: 'fraction', value: '23' },
{ type: 'literal', value: ' ' },
{ type: 'unit', value: 'MB' },
{ type: 'literal', value: '.' }

let submitButton = new SomeUnstringifiableClass();
"Ready? Then {$submitButton}!"

{ type: 'literal', value: 'Ready? Then  ' },
{ type: 'opaque', value: SomeUnstringifiableClass {} },
{ type: 'literal', value: '!' }

[stasm]

(Sorry for the notification noise for everyone whom GitHub autoflagged for a review. I accidentally opened this PR against master which triggered the code owners reviews.)