refactor: rename BugsnagCrashReport to BugsnagEvent

[robinmacharg]

Goal

Rename the BugsnagCrashReport class to BugsnagEvent as a more accurate description of purpose and for greater consistency across Notifier platforms.

Design

Simple class rename.

Changeset

All mentions of BugsnagCrashReport changed to BugsnagEvent.

Tests

Tests were included in the rename, tests run manually and passed.

Review

Outstanding Questions

KSCrashReport mentions have been left as-is to reflect the origin of the BSG naming.

• This pull request is ready for:
  • Initial review of the intended approach, not yet feature complete
  • Structural review of the classes, functions, and properties modified
  • Final review

• The correct target branch has been selected (master for fixes, next for
  features)
• Consistency across platforms for structures or concepts added or modified
• Consistency between the changeset and the goal stated above
• Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• Usage friction - is the proposed change in usage cumbersome or complicated?
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Concurrency concerns - if components are accessed asynchronously, what issues will arise
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language