Make most lexer errors recoverable

[mikmart]

The idea is that when the lexer encounters an error it can tell the compiler whether compilation can continue or not based on the Err variant of Result rather than a simple Option:

pub type Result = core::result::Result<(), ErrorKind>;

#[derive(Clone, Copy)]
pub enum ErrorKind {
    Error, // An (invalid) token is available; compilation can continue.
    Fatal, // No token is available; compilation must be terminated.
}

However there was a bad interaction with the backtracking we do in the compiler, which caused duplicate errors being reported when tokens were re-lexed. To avoid that, I implemented peek_token() and used it instead where possible.

Closes #205.

[mikmart]

Before:

$ build/b tests/lexer_errors.b
INFO: Compiling files tests/lexer_errors.b ./libb/all.b ./libb/gas-x86_64-linux.b
tests/lexer_errors.b:5:5: LEXER ERROR: Empty character literal

After, without peeking:

$ build/b tests/lexer_errors.b
INFO: Compiling files tests/lexer_errors.b ./libb/all.b ./libb/gas-x86_64-linux.b
tests/lexer_errors.b:5:5: LEXER ERROR: Empty character literal
tests/lexer_errors.b:5:5: LEXER ERROR: Empty character literal
tests/lexer_errors.b:5:5: LEXER ERROR: Empty character literal
tests/lexer_errors.b:6:5: LEXER ERROR: Character literal contains more than two characters
tests/lexer_errors.b:6:5: LEXER ERROR: Character literal contains more than two characters
tests/lexer_errors.b:6:5: LEXER ERROR: Character literal contains more than two characters
tests/lexer_errors.b:9:5: LEXER ERROR: Integer literal overflow
tests/lexer_errors.b:9:5: LEXER ERROR: Integer literal overflow
tests/lexer_errors.b:9:5: LEXER ERROR: Integer literal overflow
tests/lexer_errors.b:10:5: LEXER ERROR: Integer literal overflow
tests/lexer_errors.b:10:5: LEXER ERROR: Integer literal overflow
tests/lexer_errors.b:10:5: LEXER ERROR: Integer literal overflow
tests/lexer_errors.b:13:7: LEXER ERROR: Unknown escape sequence starting with `f`
tests/lexer_errors.b:13:11: LEXER ERROR: Unknown escape sequence starting with `b`
tests/lexer_errors.b:13:15: LEXER ERROR: Unknown escape sequence starting with `b`
tests/lexer_errors.b:13:7: LEXER ERROR: Unknown escape sequence starting with `f`
tests/lexer_errors.b:13:11: LEXER ERROR: Unknown escape sequence starting with `b`
tests/lexer_errors.b:13:15: LEXER ERROR: Unknown escape sequence starting with `b`
tests/lexer_errors.b:13:7: LEXER ERROR: Unknown escape sequence starting with `f`
tests/lexer_errors.b:13:11: LEXER ERROR: Unknown escape sequence starting with `b`
tests/lexer_errors.b:13:15: LEXER ERROR: Unknown escape sequence starting with `b`

After, with peeking:

$ build/b tests/lexer_errors.b
INFO: Compiling files tests/lexer_errors.b ./libb/all.b ./libb/gas-x86_64-linux.b
tests/lexer_errors.b:5:5: LEXER ERROR: Empty character literal
tests/lexer_errors.b:6:5: LEXER ERROR: Character literal contains more than two characters
tests/lexer_errors.b:9:5: LEXER ERROR: Integer literal overflow
tests/lexer_errors.b:10:5: LEXER ERROR: Integer literal overflow
tests/lexer_errors.b:13:7: LEXER ERROR: Unknown escape sequence starting with `f`
tests/lexer_errors.b:13:11: LEXER ERROR: Unknown escape sequence starting with `b`
tests/lexer_errors.b:13:15: LEXER ERROR: Unknown escape sequence starting with `b`