refactor!: rename "gateway" to "sender" everywhere

[aasseman]

Fixes #188

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🤖 Cargo Audit Report 🤖

(Empty means OK! 👍)

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Pull Request Test Coverage Report for Build 6871569828

• 26 of 28 (92.86%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 82.176%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tap_core/src/adapters/test/escrow_adapter_mock.rs	18	20	90.0%
<!--	Total:	26	28	92.86%	-->

Totals
Change from base Build 6773696788:	0.0%
Covered Lines:	1231
Relevant Lines:	1498

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