Merge #1090 #1097 #1098 #1099 #1101

1090: macros: add concat! macro r=philberty a=liushuyu

- extracts parenthesis-matching logic into a function
- adds `concat!` macro

1097: Support mangling *const ptr and slices like *const [T] r=philberty a=philberty

The legacy mangling scheme needs to convert the canonical path containing
* for pointers and the [] brackets representing slices into:

  * = $BP$
  [ = $u5b$
  ] = $u5d$

These symbols are not allowed in asm symbols.

Addresses #849


1098: Ensure unsize method resolutions actually unsize r=philberty a=philberty

This was a typo when unsized method resolution was added, where the
adjustment was wrongly marked as an indirection. The enum is required so
that the code generation adjustment takes place.

Addresses #849

1099: Fix bad inherent overlap error r=philberty a=philberty

When we examine HIR::ImplBlock's we determine if an impl might overlap
another impl based on the Self type. So for example you might have a
generic structure Foo<T>(T), and an associated impl block for Foo<i32>, but
then go on to define an associated impl of Foo<T> the generic one will
overlap any associated impl hiding the generic implementation.

In this case we have two generic impl blocks

  *const [T]
  *const T

This means the *const T might overlap with the slice one since it is
generic. As bjorn3 pointed out in #1075 , the correct implementation is to
observe that [T] is constrained by size but untill we have the auto trait
of Sized we must example the two generic impls and just determine that
they are not-equal so for now this is the best implementation we can do.

Fixes #1075 


1101: Add helper as_string for DefIds r=philberty a=philberty

This just adds a useful helper to as_string DefId's directly

Co-authored-by: liushuyu <liushuyu011@gmail.com>
Co-authored-by: Philip Herron <philip.herron@embecosm.com>