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This implements dependency resolution to sort amount generator lines
so that a base_amount is never contributed after it was applied.

Before, it was required to sort manually using int_index or float_index, which
can be difficult for a human when there are many lines spreaded over different
containers (which are merged by composition). Another problematic case is when
a set of lines is configured by a user (like discounts & fees) and must all be
applied before other lines (taxes) that are installed elsewhere by the
developer: how to reliably make sure the latter have index values that are
already greater than those entered by the user ?

Setting int_index or float_index is now only useful for lines:
- with same reference: only the maching one with lowest index is taken
  into account (commit 68ec6bda)
- applying to intermediate values of some base_amount
  (commit 10be013b...