feat: allow empty dict creation via dict! {}

[rracariu]

Refactor dict macro syntax for consistency and add test for empty dict

[github-actions]

Coverage after merging feat/empty-dict-macro into master will be

82.10%

Coverage Report for Changed Files

File	Stmts	Branches	Funcs	Lines	Uncovered Lines
src/haystack/val
dict.rs	92.85%	100%	93.33%	92.75%	206, 232, 448, 463–466, 468, 473, 487, 496–498, 505–507, 640–642, 644–646, 648–650, 763–765, 781, 788–790, 909, 942
dis_macro.rs	97.67%	100%	100%	97.27%	50–52