rework async function semantics

[andrewrk]

This is #2377 as well as #1778.

checklist:

• zig fmt support for the syntax
• compile error (instead of crashing) for trying to get @Frame of generic function
• compile error (instead of crashing) for trying to async call and passing @Frame of wrong function
• implicit cast of normal function to async function should be allowed when it is inferred to be async
• compile error for error: expected anyframe->T, found 'anyframe'
• compile error for error: expected anyframe->T, found 'i32'
• peer type resolution of *@Frame(func) and anyframe
• peer type resolution of *@Frame(func) and anyframe->T when the return type matches
• for loops need to spill the index. other payload captures probably also need to spill
• const result = (await a) + (await b); this causes "Instruction does not dominate all uses" - need spill
• @typeInfo for @Frame(func)
• returning a value from within a suspend block
• make resuming inside a suspend block, with nothing after it, a must-tail call.
• make sure there are safety tests for all the new safety features (search the new PanicFnId enum values)
• compile error for casting a function to a non-async function pointer, but then later it gets inferred to be an async function
• compile error for copying a frame
• compile error for resuming a const frame pointer
• runtime safety enabling/disabling scope has to be coordinated across resume/await/calls/return
• calling a generic function which is async
• make sure await @asyncCall and await async are handled correctly.
• allow @asyncCall with a real @Frame(func) (the point of this is result pointer)
• when there are multiple calls to async functions in a function, reuse the same frame buffer, so that the
  needed bytes is equal to the largest callee's frame
• if an async function is never called with async then a few optimizations can be made:
  • the return does not need to be atomic
  • it can be assumed that these are always available: the awaiter ptr, return ptr if applicable,
    error return trace ptr if applicable.
  • it can be assumed that it is never cancelled
• fix the debug info for variables of async functions
• re-order the frame fields for less memory usage for async calls of blocking functions
• await should not be a suspend point when used on a @Frame(func) and func is blocking.

[andrewrk]

Follow-up items are in issue #3063.