Introduce funding contribution builder API

This commit introduces a `FundingBuilder`, following the builder
pattern, to iteratively construct a `FundingContribution`. It supports
adding/removing value (which may trigger a coin selection attempt),
adding/remove outputs, and amending prior contributions (which was
previously not possible).

A `FundingBuilder` may be obtained in two ways: via
`FundingTemplate::contribution_builder` (which resumes from the prior
contribution to amend if available) or
`FundingTemplate::without_prior_contribution_builder` to start from
scratch and fully replace the prior contribution.

When `FundingBuilder::build` is called, we attempt to build the
contribution iteratively such that coin selection is our last resort.

The splice-related convenience methods on `FundingTemplate` now map to
the `FundingBuilder` under the hood.

Author: wpaulino