Add a comment to each of supported tokens
Treat non-numeric values correctly on conversions

Used to assume that all variables are of string type.
Extend parser to support non-negative numbers

For now, negative numbers as well as number signs are not
supported.
Get rid of target buffer concept in parser

It's bad idea if there is more than one type of variables.
Extract str_to_int() function

Converts string into integer handling underflow and overflow.
Extend parser to parse plus and minus signs
Add engine/var.c:var_to_integer() function

Converts variable to an integer.
Automatically parse strings into numbers for - & +
Document integer number constants and unary +/-
Merge branch 'add-integer-constants'

Extend parser to support integer number constants and unary minus and
plus operators.
Don't stringize expressions without concatenation

Preserve type of the first term.
Introduce tokens for <, >, <= and >= operators
Add comparison operators (<, <=, >=, >)
Fix evaluation of :execute and :echo arguments

Need to skip whitespace to guarantee moving throw the input string,
otherwise the algorithm ends up in infinite loop.
Document adding comparison operators
Merge branch 'add-number-comparison-operators'

Extend parser to support <, <=, >= and > comparison operators for
strings and integers.
Add count tests for engine/keys unit

The most important one is test_huge_number_get_intmax(), because
it covers one of previous bugs that led to a segfault (in application,
not just in keys module).
Add keys count test with INT_MAX as count
Merge branch 'add-missing-count-tests'

Tests for count before keys were missing.  Especially that test
about overflow issue.