String Instructions

The following instructions are modified to act on strings:
- INDEX: Now pulls a char out at index
- SUBSL: Now pulls a substring out of a source string
- INSER: Now inserts a char into a string at index
- LENGTH: Now returns length of a string

In addition to the above instructions, additional instructions
are now implemented to handle strings:
- CONCAT: appends a string onto another string
- S_APPEND: appends a char on to the end of a string

Fixes: #38

Signed-off-by: Ava Affine <ava@sunnypup.io>

hyphae/instructions.toml

@@ -312,3 +312,15 @@ name = "cdr"
 args = ["list"]
 output = "returns last element in cons cell"
 description = "takes an AST and returns last element in top level cons cell"
+
+[[instructions]]
+name = "concat"
+args = ["string_l", "string_r"]
+output = "string_l+string_r"
+description = "concatenates string r to string l and returns a new string"
+
+[[instructions]]
+name = "s_append"
+args = ["parent", "child"]
+output = ""
+description = "append in place child character into parent string"