Removing secfrac and tzmin components

[dgkf]

Closes #25

A pretty comprehensive refactor of the internal representation to avoid redundancy in sec/secfrac and tzhour/tzmin.

Lots of coercion, formatting and tests were reworked to support this.

[codecov-commenter]

Codecov Report

Merging #43 (78d76bb) into main (e9d43fa) will decrease coverage by 0.50%. The diff coverage is 83.33%.

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
- Coverage   63.23%   62.73%   -0.51%     
==========================================
  Files          41       41              
  Lines         914      899      -15     
==========================================
- Hits          578      564      -14     
+ Misses        336      335       -1     

Impacted Files	Coverage Δ
R/reflow_fields.R	75.00% <ø> (-3.05%)	:arrow_down:
R/utils.R	60.22% <ø> (ø)
R/class_partial_time_compat_vctrs.R	4.76% <25.00%> (-2.22%)	:arrow_down:
R/impute.R	49.23% <25.00%> (-3.80%)	:arrow_down:
R/class_partial_time_compat_lubridate.R	34.21% <66.66%> (+1.65%)	:arrow_up:
R/to_gmt.R	61.53% <75.00%> (ø)
R/class_partial_time_format.R	96.92% <87.50%> (-1.42%)	:arrow_down:
R/class_partial_time_coercion.R	85.50% <96.00%> (+0.43%)	:arrow_up:
R/class_partial_time.R	88.88% <100.00%> (-2.78%)	:arrow_down:
R/class_partial_time_utils.R	93.75% <100.00%> (ø)
... and 3 more

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