Incorrect context type in handlers

[Raiondesu]

Describe the bug
The type of the event context (TEventContext) is not inferred correctly in event handlers passed to subscribe function.

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To Reproduce

import { eventMap, on } from 'eventhoven';

const map = eventMap({
  event(_): 42 { return 42; }
});

on(map)('event')(ctx => {
  ctx.event; // string | number | symbol
  ctx.unsubscribe // TUnsubscribe
});

Expected behavior

import { eventMap, on } from 'eventhoven';

const map = eventMap({
  event(_): 42 { return 42; }
});

on(map)('event')(ctx => {
  ctx.event; // 'event'
  ctx.unsubscribe // TUnsubscribe<'event'>
});

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Environment (please complete the following information):

• OS: Windows
• Platform:
  • Browser - any
  • Node 12+
• eventhoven version: v1.0.0

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