Update json.rs

Treemap should be BTreeMap
core: Removed a shadowed, unused definition of `debug_assert!`.
Fix a typo
doc: remove repeated info
Add a test case for issue 18906

Closes issue #18906
doc: surround with symbols, like it should
hashmap: Fix the example using derived Hash + Eq

The example derived Hash + Eq on a type that was used as *values* for
a hashmap.. for the example to make sense, we have to use a custom *key*
type.

Write a slightly more involved example, still using Vikings, but this
time as key.

I preferred using String over &str here, since that's the typical usage
and we might want to lead users down that path.
Rename remaining hashmap and hashtable iterators to match naming
conventions.

This is a [breaking-change].
Fix backtrace demangling

Closes #20209
librustc_trans: Remove some dead code now that procs are gone.
trans: Remove is_lang_item from base::invoke

Removes a FIXME on closed issue #15064. This flag is no
longer needed or used since reflection is gone.
Merge pull request #20217 from luqmana/pc

librustc_trans: Remove some dead code now that procs are gone.

Reviewed-by: sfackler
Merge pull request #19457 from reem/remove-is-lang-item

trans: Remove is_lang_item from base::invoke

Reviewed-by: alexcrichton
Merge pull request #20182 from brianloveswords/patch-2

Update json.rs (Docs)

Reviewed-by: alexcrichton
Merge pull request #20191 from lifthrasiir/double-debug_assert

core: Removed a shadowed, unused definition of `debug_assert!`.

Reviewed-by: cmr
Merge pull request #20195 from nagisa/unused-typo

Fix a typo

Reviewed-by: alexcrichton
Merge pull request #20205 from tshepang/patch-4

doc: remove repeated info

Reviewed-by: steveklabnik
Merge pull request #20207 from jroesch/issue-18906-testcase

Add a test case for issue 18906

Reviewed-by: alexcrichton
Merge pull request #20210 from tshepang/patch-5

doc: surround with symbols, like it should

Reviewed-by: steveklabnik
Merge pull request #20214 from bluss/fix-hashmap-example

hashmap: Fix the example using derived Hash + Eq

Reviewed-by: alexcrichton