Fixes rust-lang#24441

The minus sign ‘−’ is the same width as the plus sign ‘+’, so the button’s transition between the two symbols will look more smooth.

Fix `make_command_line` for the case of backslashes at the end of an
argument requiring quotes.  We must encode the command and arguments
such that `CommandLineToArgvW` recovers them in the spawned process.
Simplify the logic by using a running count of backslashes as they
are encountered instead of looking ahead for quotes following them.

Extend `test_make_command_line` to additionally cover:

* a leading quote in an argument that requires quotes,
* a backslash before a quote in an argument that requires quotes,
* a backslash at the end of an argument that requires quotes, and
* a backslash at the end of an argument that does not require quotes.

So that if people accidentally delete the character, they won’t re-type it as a hyphen, which would cause bugs.

I changed ‘+’ too, even though it won’t be re-typed incorrectly, so that it is easier to see when plus is used as a symbol for the button, and when it is used as an operator in code. It also makes it clearer that the use of an entity for minus is on purpose, so people won’t be tempted to replace the entity incorrectly with a hyphen character.

Remove the name "multi-line string literal" since the rule appears to affect each line-break individually rather than the whole string literal. Re-word, and remove the stray reference to raw strings.

…mmand-line, r=alexcrichton

Fix `make_command_line` for the case of backslashes at the end of an
argument requiring quotes.  We must encode the command and arguments
such that `CommandLineToArgvW` recovers them in the spawned process.
Simplify the logic by using a running count of backslashes as they
are encountered instead of looking ahead for quotes following them.

Extend `test_make_command_line` to additionally cover:

* a leading quote in an argument that requires quotes,
* a backslash before a quote in an argument that requires quotes,
* a backslash at the end of an argument that requires quotes, and
* a backslash at the end of an argument that does not require quotes.

Clarify type inference of floating-point literals

The minus sign ‘−’ is the same width as the plus sign ‘+’, so the button’s transition between the two symbols will look slightly smoother.

If you don’t want to use literal Unicode characters, I can change ‘−’ to `\u2212`. I’m not starting with that suggestion because ‘−’ is easier to read and understand, and if I used `\u2212`, it would probably be necessary to also comment the usage on each line to explain what character is being used.

Fixes rust-lang#24441 

Preview:
![new-trait-color](https://cloud.githubusercontent.com/assets/346530/7331922/d6cbcf72-eb58-11e4-8a1d-4ca5e3683ed2.png)

Improve example for as_string and add example for as_vec

Provide a better example of `as_string` / `DerefString`'s unique capabilities.
Use an example where (for an unspecified reason) you need a &String, and
show how `as_string` solves the problem without needing an allocation.

A user in IRC was having trouble because they used `main.rs` when they were trying
to migrate a library. The `[[lib]]` key is not easily found, and the `main.rs`/`lib.rs`
distinction doesn't seem to exist in trpl


r? @steveklabnik

Remove the name "multi-line string literal" since the rule appears to affect each line-break individually rather than the whole string literal. Re-word, and remove the stray reference to raw strings.

…eklabnik