Merge #2911

2911: Implement collecting errors while tokenizing r=matklad a=Veetaha

Now we are collecting errors from `rustc_lexer` and returning them in `ParsedToken { token, error }` and `ParsedTokens { tokens, errors }` structures **([UPD]: this is now simplified, see updates bellow)**.

The main changes are introduced in `ra_syntax/parsing/lexer.rs`. It now exposes the following functions and types:

```rust
pub fn tokenize(text: &str) -> ParsedTokens;
pub fn tokenize_append(text: &str, parsed_tokens_to_append_to: &mut ParsedTokens);
pub fn first_token(text: &str) -> Option<ParsedToken>; // allows any number of tokens in text
pub fn single_token(text: &str) -> Option<ParsedToken>; // allows only a single token in text

pub struct ParsedToken  { pub token: Token,       pub error: Option<SyntaxError> }
pub struct ParsedTokens { pub tokens: Vec<Token>, pub errors: Vec<SyntaxError>   }

pub enum TokenizeError { /* Simple enum which reflects rustc_lexer tokenization errors */ }
```
In the first commit I implemented it with iterators, but then decided that since this crate is ad hoc for `rust-analyzer` and we clearly see the places of its usage it would be better to simplify it to vectors.

This is currently WIP, because I want to add tests for error messages generated by the lexer.
I'd like to listen to you thoughts how to define these tests in `ra_syntax/test-data` dir.

Related issues: #223 

**[UPD]**

After the PR review the API was simplified:
```rust
pub fn tokenize(text: &str) -> (Vec<Token>, Vec<SyntaxError>);
// Both lex functions do not check for unescape errors
pub fn lex_single_syntax_kind(text: &str) -> Option<(SyntaxKind, Option<SyntaxError>)>;
pub fn lex_single_valid_syntax_kind(text: &str) -> Option<SyntaxKind>;

// This will be removed in the next PR in favour of simlifying `SyntaxError` to `(String, TextRange)`
pub enum TokenizeError { /* Simple enum which reflects rustc_lexer tokenization errors */ }

// this is private, but may be made public if such demand would exist in future (least privilege principle)
fn lex_first_token(text: &str) -> Option<(Token, Option<SyntaxError>)>;
```

Co-authored-by: Veetaha <[email protected]>

1 files changed, 64 insertions, 0 deletions

diff --git a/crates/ra_syntax/test_data/lexer/ok/0011_keywords.txt b/crates/ra_syntax/test_data/lexer/ok/0011_keywords.txt
new file mode 100644
index 000000000..22c00eefb
--- /dev/null
+++ b/crates/ra_syntax/test_data/lexer/ok/0011_keywords.txt
@@ -0,0 +1,64 @@
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