warn on unknown fields and confusable targets

[Emilgardis]

also fixes #643

[Emilgardis]

on

[build]
default-target = "x86_64-unknown-linux-gnu"

[target.x86-64-unknown-linux-gnu]
image = "aaa:aa"
aaaa = "a"
aaaaa = "a"

this will warn

Warning: found unused key(s) in in Cross configuration:
 > target.x86-64-unknown-linux-gnu.aaaa, target.x86-64-unknown-linux-gnu.aaaaa
Warning: a target named "x86-64-unknown-linux-gnu" is mentioned in the Cross configuration, but the current specified target is "x86_64-unknown-linux-gnu".
 > Is the target misspelled in the Cross configuration?

The formatting should probably be a bit standardized :)

[Emilgardis]

rebased the pr to incorporate the suggestions

edit:
note to self: don't rebase without having had your coffee

[Emilgardis]

bors r=reitermarkus

[bors]

Build succeeded:

• conclusion