Tracking issue for RFC 2025: Enabled nested method calls

[aturon]

This is a tracking issue for the RFC "Enable nested method calls" (rust-lang/rfcs#2025).

Steps:

• Implement the RFC
  • Implementation specifically is tracked in MIR Borrowck: two-phased borrows #46037
• Adjust documentation (see instructions on forge)
• Stabilization PR (see instructions on forge)

Issues and questions to be resolved prior to stabilization:

• as currently implemented, the two-phase borrows apply more broadly than the RFC text permitted for initial release, limit two phase borrows to method-call autorefs #46747

[nikomatsakis]

closing in favor of #43234

[pnkfelix]

(I'm a little worried that closing this in favor of #43234 is going to increase the chance that we either overlook or prepetually delay resolving #46901...)