feat(boa): implements at method for string

[neeldug]

This Pull Request relates to #13.

It changes the following:

• implements at method for strings

[neeldug]

Test262 conformance changes:

Test result	master count	PR count	difference
Total	78,897	78,897	0
Passed	26,974	26,996	+22
Ignored	15,616	15,616	0
Failed	36,307	36,285	-22
Panics	0	0	0
Conformance	34.19%	34.22%	+0.03%

Fixed tests:

test/built-ins/String/prototype/at/index-argument-tointeger.js [strict mode] (previously Failed)
test/built-ins/String/prototype/at/index-argument-tointeger.js (previously Failed)
test/built-ins/String/prototype/at/index-non-numeric-argument-tointeger-invalid.js [strict mode] (previously Failed)
test/built-ins/String/prototype/at/index-non-numeric-argument-tointeger-invalid.js (previously Failed)
test/built-ins/String/prototype/at/returns-code-unit.js [strict mode] (previously Failed)
test/built-ins/String/prototype/at/returns-code-unit.js (previously Failed)
test/built-ins/String/prototype/at/name.js [strict mode] (previously Failed)
test/built-ins/String/prototype/at/name.js (previously Failed)
test/built-ins/String/prototype/at/returns-item.js [strict mode] (previously Failed)
test/built-ins/String/prototype/at/returns-item.js (previously Failed)
test/built-ins/String/prototype/at/returns-item-relative-index.js [strict mode] (previously Failed)
test/built-ins/String/prototype/at/returns-item-relative-index.js (previously Failed)
test/built-ins/String/prototype/at/index-non-numeric-argument-tointeger.js [strict mode] (previously Failed)
test/built-ins/String/prototype/at/index-non-numeric-argument-tointeger.js (previously Failed)
test/built-ins/String/prototype/at/returns-undefined-for-out-of-range-index.js [strict mode] (previously Failed)
test/built-ins/String/prototype/at/returns-undefined-for-out-of-range-index.js (previously Failed)
test/built-ins/String/prototype/at/return-abrupt-from-this.js [strict mode] (previously Failed)
test/built-ins/String/prototype/at/return-abrupt-from-this.js (previously Failed)
test/built-ins/String/prototype/at/prop-desc.js [strict mode] (previously Failed)
test/built-ins/String/prototype/at/prop-desc.js (previously Failed)
test/built-ins/String/prototype/at/length.js [strict mode] (previously Failed)
test/built-ins/String/prototype/at/length.js (previously Failed)

[Razican]

Note that this doesn't close #13, since there are more methods left to implement.

[Razican]

Looks pretty good to me. Check my comments. Note that this is a "proposal", and therefore not part of #13, and no other JS engine is implementing it. I want to know what @jasonwilliams thinks about adding these things to Boa.

[neeldug]

Looks pretty good to me. Check my comments. Note that this is a "proposal", and therefore not part of #13, and no other JS engine is implementing it. I want to know what @jasonwilliams thinks about adding these things to Boa.

I mainly added it thinking it was failing the test262 suite, but I see what you mean.

[HalidOdat]

Since it is in test262, its probably going to be added to the language eventualy, but maybe we should wait till then.

[jasonwilliams]

Looks pretty good to me. Check my comments. Note that this is a "proposal", and therefore not part of #13, and no other JS engine is implementing it. I want to know what @jasonwilliams thinks about adding these things to Boa.

Its ok with me, I don't mind seeing proposals like this one being added as its quite likely to make it in