[Feature] String parsing

[damirka]

Motivation

Closes #943.
Closes #944.

Test Plan

Features parser tests for string literals.

Related PRs

This PR is targeted to #957.

[codecov]

Codecov Report

Merging #974 (a3416c1) into feature/char-type (cd95f48) will increase coverage by 0.04%.
The diff coverage is 76.41%.

@@                  Coverage Diff                  @@
##           feature/char-type     #974      +/-   ##
=====================================================
+ Coverage              75.25%   75.29%   +0.04%     
=====================================================
  Files                    434      435       +1     
  Lines                  17425    17552     +127     
=====================================================
+ Hits                   13113    13216     +103     
- Misses                  4312     4336      +24     

Impacted Files	Coverage Δ
asg/src/expression/constant.rs	65.92% <0.00%> (-0.75%)	⬇️
ast/src/errors/reducer.rs	5.55% <0.00%> (-1.59%)	⬇️
input/src/errors/parser.rs	26.53% <0.00%> (-2.36%)	⬇️
input/src/expressions/expression.rs	6.06% <0.00%> (-0.61%)	⬇️
parser/src/tokenizer/mod.rs	97.50% <ø> (-0.04%)	⬇️
input/src/expressions/string_expression.rs	20.00% <20.00%> (ø)
ast/src/expression/value.rs	72.97% <28.57%> (-10.90%)	⬇️
compiler/src/phases/reducing_director.rs	48.07% <60.00%> (-0.41%)	⬇️
input/src/values/char_types.rs	85.07% <72.22%> (+17.76%)	⬆️
parser/src/tokenizer/token.rs	90.52% <75.00%> (+2.29%)	⬆️
... and 16 more

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[gluax]

Updated parsing to leverage the char parsing.
Now closes #951.

Wanted to bring up an interesting point I don't think we discussed as well. What about empty string? An empty string in Leo would resolve to [char; 0] which we don't allow.

[acoglio]

@gluax I think you are right that we didn't discuss the empty string issue at meetings, but it is mentioned in the RFC -- 4th paragraph in https://github.com/AleoHQ/leo/blob/master/docs/rfc/001-initial-strings.md#strings. It does feel strange at first, but then I think it makes sense once the flattening to R1CS is taken into consideration.

[gluax]

@acoglio Yea, I saw it in the RFC and your ABNF. I suppose I was asking since we never really discussed it and went along with it. It is probably not an issue for MVP purposes as well—just something to consider in the future.

[gluax]

After the recent commit:
Closes #947.
Closes #949.

[acoglio]

Looks good.

[damirka]

@gluax I've added max value (127) for \x escape and tests for it. Overall this PR looks complete, I've run few programs locally and strings are working, inputs as well. Great work!

@acoglioI think we finally can merge it into char.