Add helper functions for invoice expiry

[benthecarman]

These should be some easy helpers for determining how much time you have to pay an invoice.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.06 ⚠️

Comparison is base (3d479c9) 91.42% compared to head (940ec03) 91.37%.

❗ Current head 940ec03 differs from pull request most recent head 5687859. Consider uploading reports for the commit 5687859 to get more accurate results

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2125      +/-   ##
==========================================
- Coverage   91.42%   91.37%   -0.06%     
==========================================
  Files         101      101              
  Lines       49552    49562      +10     
  Branches    49552    49562      +10     
==========================================
- Hits        45303    45285      -18     
- Misses       4249     4277      +28     

Impacted Files	Coverage Δ
lightning-invoice/src/lib.rs	79.94% <0.00%> (-1.13%)	⬇️

... and 4 files with indirect coverage changes

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[benthecarman]

Responded to @dunxen and @valentinewallace's review

[valentinewallace]

CI is sad:

error[E0599]: no associated item named `ZERO` found for struct `Duration` in the current scope
    --> lightning-invoice/src/lib.rs:1261:25
     |
1261 |             .unwrap_or(Duration::ZERO)
     |                                  ^^^^ associated item not found in `Duration`

[benthecarman]

CI is sad:

Fixed, apparently that was added in 1.56 :/